Forensic Science

Respond to one of the following:

Option 1:The following pieces of evidence were found at separate explosion sites. For each item, indicate whether the explosion was caused by low or high explosives, and explain your answers: lead azide residues; nitrocellulose residues; ammonium nitrate residues; scraps of prima-cord; potassium chlorate residues.

Option 2: What color test or tests would you run on a suspect ample to test for evidence of each of the following explosives? Explain your answers: tetryl; TNT; chlorate; nitrocellulose.

Option 3: Matt is collecting evidence from the site of an explosion. Arriving at the scene, he immediately proceeds to look for the crater caused by the blast. After finding the crater, he picks through the debris at the site by hand, looking for evidence of detonators or foreign materials. Matt collects loose soil and debris from the immediate area, placing the smaller bits in paper folded into a druggist fold. Larger items he stores in plastic bags for transportation to the laboratory. What mistakes, if any did Matt make in collecting and storing this evidence.

Option 4: While searching a murder scene, you find the following items that you believe may contain latent fingerprints. Indicate whether prints on each item should be developed using fingerprint powder or chemicals: a leather sofa; a mirror; a painted wooden knife handle; blood-soaked newspapers; a revolver.

Option 5: Criminalist Frank is using digital imaging to enhance latent fingerprints. Indicate which features of digital imaging he would most likely use for each of the following tasks:

1. isolating part of a print and enlarging it for closer examination
2. increasing the contrast between a print and the background surface on which it is located
3. examining two prints that overlap each other.

Option 6: Briefly explain the chemistry of an explosion. What is the difference between a low explosive and a high explosive?

Option 7: Describe the process of collecting evidence of explosives?

Option 8: What is the difference between screening and confirmatory tests? Why are both needed? What is a taggant? Which countries are presently using them?

Option 9: List the three principles of fingerprints and briefly describe them.

Option 10: Describe the procedures to locate and develop fingerprints.

Option 12: Describe the procedures to preserve fingerprints.

ONLY HAVE TO BE 150 WORDS OR MORE MAKE SURE TO SITE YOUR WORK WITH REFERENCES

The Oklahoma City Bombing

It was the biggest act of mass murder in U.S. history. On a sunny spring morning in April 1995, a Ryder rental truck pulled into the parking area of the Alfred P. Murrah federal building in Oklahoma City. The driver stepped down from the truck’s cab and casually walked away. Minutes later, the truck exploded into a fireball, unleashing enough energy to destroy the building and kill 168 people, including 19 children and infants in the building’s day care center.

Later that morning, an Oklahoma Highway Patrol officer pulled over a beat-up 1977 Mercury Marquis being driven without a license plate. On further investigation, the driver, Timothy McVeigh, was found to be in possession of a loaded firearm and charged with transporting a firearm. At the explosion site, remnants of the Ryder truck were located and the truck was quickly traced to a renter—Robert Kling, an alias for Timothy McVeigh. Coincidentally, the rental agreement and McVeigh’s driver’s license both used the address of McVeigh’s friend, Terry Nichols.

Investigators later recovered McVeigh’s fingerprint on a receipt for 2,000 pounds of ammonium nitrate, a basic

explosive ingredient. Forensic analysts also located PETN residues on the clothing McVeigh wore on the day of his arrest. PETN is a component of detonating cord.

After three days of deliberation, a jury declared McVeigh guilty of the bombing and sentenced him to die by lethal injection.

headline news

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After studying this chapter you should be able to: • Understand how explosives are classified

• List some common commercial, homemade, and military explosives

• Describe how to collect physical evidence at the scene of an explosion

• Describe laboratory procedures used to detect and identify explosive residues.

forensic investigation of explosions

black powder deflagration detonating cord detonation explosion high explosive low explosive oxidizing agent primary explosive safety fuse secondary explosive smokeless powder

(double-base) smokeless powder

(single-base)

KEY TERMS

> > > > > > > > > > > > chapter15

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oxidizing agent A substance that supplies oxygen to a chemical reaction

explosion A chemical or mechanical action caused by combustion, accompanied by creation of heat and rapid expansion of gases

374 CHAPTER 15

Explosions and Explosives The ready accessibility of potentially explosive laboratory chemicals, dynamite, and, in some countries, an assortment of military explosives has provided the criminal element of society with a lethal weapon. Unfortunately for society, explosives have become an attractive weapon to crim- inals bent on revenge, destruction of commercial operations, or just plain mischief.

Although politically motivated bombings have received considerable publicity worldwide, in the United States most bombing incidents are perpetrated by isolated individuals rather than by organized terrorists. These incidents typically involve homemade explosives and incendiary devices. The design of such weapons is limited only by the imagination and ingenuity of the bomber.

Like arson investigation, bomb investigation requires close cooperation of a group of highly specialized individuals trained and experienced in bomb disposal, bomb-site investigation, foren- sic analysis, and criminal investigation. The criminalist must detect and identify explosive chem- icals recovered from the crime scene as well as identify the detonating mechanisms. This special responsibility concerns us for the remainder of this chapter.

The Chemistry of Explosions Like fire, an explosion is the product of combustion accompanied by the creation of gases and heat. However, the distinguishing characteristic of an explosion is the rapid rate of the reaction. The sudden buildup of expanding gas pressure at the origin of the explosion produces the violent physical disruption of the surrounding environment.

Our previous discussion of the chemistry of fire referred only to oxidation reactions that rely on air as the sole source of oxygen. However, we need not restrict ourselves to this type of situa- tion. For example, explosives are substances that undergo a rapid exothermic oxidation reaction, producing large quantities of gases. This sudden buildup of gas pressure constitutes an explosion. Detonation occurs so rapidly that oxygen in the air cannot participate in the reaction; thus, many explosives must have their own source of oxygen.

Chemicals that supply oxygen are known as oxidizing agents. One such agent is found in black powder, a low explosive, which is composed of a mixture of the following chemical ingredients:

75 percent potassium nitrate (KNO3) 15 percent charcoal (C) 10 percent sulfur (S)

In this combination, oxygen containing potassium nitrate acts as an oxidizing agent for the char- coal and sulfur fuels. As heat is applied to black powder, oxygen is liberated from potassium nitrate and simultaneously combines with charcoal and sulfur to produce heat and gases (symbolized by �), as represented in the following chemical equation:

3C � S � 2KNO3 � carbon sulfur potassium nitrate yields

3CO2� � N2� � K2S carbon dioxide nitrogen potassium sulfide

Some explosives have their oxygen and fuel components combined within one molecule. For example, the chemical structure of nitroglycerin, the major constituent of dynamite, combines carbon, hydrogen, nitrogen, and oxygen:

HHH ƒ ƒ ƒ

H ¬ C ¬ C ¬ C ¬ H ƒ ƒ ƒ

NO2 NO2 NO2

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When nitroglycerin detonates, large quantities of energy are released as the molecule decomposes, and the oxygen recombines to produce large volumes of carbon dioxide, nitrogen, and water.

Consider, for example, the effect of confining an explosive charge to a relatively small, closed container. On detonation, the explosive almost instantaneously produces large volumes of gases that exert enormously high pressures on the interior walls of the container. In addition, the heat energy released by the explosion expands the gases, causing them to push on the walls with an even greater force. If we could observe the effects of an exploding lead pipe in slow motion, we would first see the pipe’s walls stretch and balloon under pressures as high as several hundred tons per square inch. Finally, the walls would fragment and fly outward in all directions. This flying debris or shrapnel constitutes a great danger to life and limb in the immediate vicinity.

On release from confinement, the gaseous products of the explosion suddenly expand and compress layers of surrounding air as they move outward from the origin of the explosion. This blast effect, or outward rush of gases, at a rate that may be as high as 7,000 miles per hour cre- ates an artificial gale that can overthrow walls, collapse roofs, and disturb any object in its path. If a bomb is sufficiently powerful, more serious damage will be inflicted by the blast effect than by fragmentation debris (see Figure 15–1).

Types of Explosives The speed at which explosives decompose varies greatly from one to another and permits their clas- sification as high and low explosives. In a low explosive, this speed is called the speed of deflagration (burning). It is characterized by very rapid oxidation that produces heat, light, and a subsonic pressure wave. In a high explosive, it is called the speed of detonation. Detonation refers to the creation of a supersonic shock wave within the explosive charge. This shock wave breaks the chemical bonds of the explosive charge, leading to the new instantaneous buildup of heat and gases.

LOW EXPLOSIVES Low explosives, such as black and smokeless powders, decompose rela- tively slowly at rates up to 1,000 meters per second. Because of their slow burning rates, they pro- duce a propelling or throwing action that makes them suitable as propellants for ammunition or skyrockets. However, the danger of this group of explosives must not be underestimated because, when any one of them is confined to a relatively small container, it can explode with a force as lethal as that of any known explosive.

Black Powder and Smokeless Powder The most widely used explosives in the low-explosive group are black powder and smokeless powder. The popularity of these two explosives is en- hanced by their accessibility to the public. Both are available in any gun store, and black powder can easily be made from ingredients purchased at any chemical supply house as well.

Black powder is a relatively stable mixture of potassium nitrate or sodium nitrate, charcoal, and sulfur. Unconfined, it merely burns; thus it commonly is used in safety fuses that carry a flame to an explosive charge. A safety fuse usually consists of black powder wrapped in a fabric or plastic casing. When ignited, a sufficient length of fuse will burn at a rate slow enough to allow a person adequate time to leave the site of the pending explosion. Black powder, like any other low explosive, becomes explosive and lethal only when it is confined.

The safest and most powerful low explosive is smokeless powder. This explosive usually consists of nitrated cotton or nitrocellulose (single-base powder) or nitroglycerin mixed with nitrocellulose (double-base powder). The powder is manufactured in a variety of grain sizes and shapes, depending on the desired application (see Figure 15–2).

Chlorate Mixtures The only ingredients required for a low explosive are fuel and a good oxidizing agent. The oxidizing agent potassium chlorate, for example, when mixed with sugar, produces a pop- ular and accessible explosive mix. When confined to a small container—for example, a pipe—and ignited, this mixture can explode with a force equivalent to a stick of 40 percent dynamite.

Some other commonly encountered ingredients that may be combined with chlorate to produce an explosive are carbon, sulfur, starch, phosphorus, and magnesium filings. Chlorate mixtures may also be ignited by the heat generated from a chemical reaction. For instance, sufficient heat can be generated to initiate combustion when concentrated sulfuric acid comes in contact with a sugar–chlorate mix.

deflagration A very rapid oxidation reaction accompanied by the generation of a low-intensity pressure wave that can disrupt the surroundings

detonation An extremely rapid oxidation reaction accompanied by a violent disruptive effect and an intense, high-speed shock wave

low explosive An explosive with a velocity of detonation less than 1,000 meters per second

black powder Normally, a mixture of potassium nitrate, carbon, and sulfur in the ratio 75/15/10

smokeless powder (double-base) An explosive consisting of a mixture of nitrocellulose and nitroglycerin

safety fuse A cord containing a core of black powder, used to carry a flame at a uniform rate to an explosive charge

smokeless powder (single-base) An explosive consisting of nitrocellulose

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high explosive An explosive with a velocity of detonation greater than 1,000 meters per second

376 CHAPTER 15

Gas–Air Mixtures Another form of low explosive is created when a considerable quan- tity of natural gas escapes into a confined area and mixes with a sufficient amount of air. If ignited, this mixture results in simultaneous combustion and sudden production of large volumes of gases and heat. In a building, walls are forced outward by the expanding gases, causing the roof to fall into the interiors, and objects are thrown outward and scattered in erratic directions with no semblance of pattern.

Mixtures of air and a gaseous fuel explode or burn only within a limited concen- tration range. For example, the concentration limits for methane in air range from 5.3 to 13.9 percent. In the presence of too much air, the fuel becomes too diluted and does not ignite. On the other hand, if the fuel becomes too concentrated, ignition is prevented because there is not enough oxygen to support the combustion.

Mixtures at or near the upper concentration limit (“rich” mixtures) explode; how- ever, some gas remains unconsumed because there is not enough oxygen to complete

the combustion. As air rushes back into the origin of the explosion, it combines with the residual hot gas, producing a fire that is characterized by a whoosh sound. This fire is often more destruc- tive than the explosion that preceded it. Mixtures near the lower end of the limit (“lean” mixtures) generally cause an explosion without accompanying damage due to fire.

HIGH EXPLOSIVES High explosives include dynamite, TNT, PETN, and RDX. They detonate almost instantaneously at rates of 1,000–8,500 meters per second, producing a smashing or shat- tering effect on their target. High explosives are classified into two groups—primary and secondary explosives—based on their sensitivity to heat, shock, or friction.

Primary explosives are ultrasensitive to heat, shock, or friction, and under normal conditions they detonate violently instead of burning. For this reason, they are used to detonate other explo- sives through a chain reaction and are often referred to as primers. Primary explosives provide the major ingredients of blasting caps and include lead azide, lead styphnate, and diazodinitrophenol (see Figure 15–3). Because of their extreme sensitivity, these explosives are rarely used as the main charge of a homemade bomb.

Secondary explosives are relatively insensitive to heat, shock, or friction, and they normally burn rather than detonate when ignited in small quantities in open air. This group comprises most high explosives used for commercial and military blasting. Some common examples of second- ary explosives are dynamite, TNT (trinitrotoluene), PETN (pentaerythritol tetranitrate), RDX (cyclotrimethylenetrinitramine), and tetryl (2,4,6-trinitrophenylmethylnitramine).

Dynamite It is an irony of history that the prize most symbolic of humanity’s search for peace—the Nobel Peace Prize—should bear the name of the developer of one of our most lethal discoveries—dynamite. In 1867, the Swedish chemist Alfred Nobel, searching for a method to desensitize nitroglycerin, found that when kieselguhr, a variety of diatomaceous earth, absorbed a large portion of nitroglycerin, it became far less sensitive but still retained its explosive force. Nobel later decided to use pulp as an absorbent because kieselguhr was a heat- absorbing material.

This so-called pulp dynamite was the beginning of what is now known as the straight dyna- mite series. These dynamites are used when a quick shattering action is desired. In addition to nitroglycerine and pulp, present-day straight dynamites also include sodium nitrate (which fur- nishes oxygen for complete combustion) and a small percentage of a stabilizer, such as calcium carbonate.

All straight dynamite is rated by strength; the strength rating is determined by the weight percentage of nitroglycerin in the formula. Thus, a 40 percent straight dynamite contains 40 percent nitroglycerin, a 60 percent grade contains 60 percent nitroglycerin, and so forth. However, the rel- ative blasting power of different strengths of dynamite is not directly proportional to their strength ratings. A 60 percent straight dynamite, rather than being three times as strong as a 20 percent, is only one and one-half times as strong (see Figure 15–4).

Ammonium Nitrate Explosives In recent years, nitroglycerin-based dynamite has all but disap- peared from the industrial explosives market. Commercially, these explosives have been replaced mainly by ammonium nitrate–based explosives, that is, water gels, emulsions, and ANFO explo- sives. These explosives mix oxygen-rich ammonium nitrate with a fuel to form a low-cost, stable explosive.

FIGURE 15–2 Samples of smokeless powders. Courtesy of ATF (Bureau of Alcohol, Tobacco, Firearms & Explosives)

primary explosive A high explosive that is easily detonated by heat, shock, or friction

secondary explosive A high explosive that is relatively insensitive to heat, shock, or friction

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Typically, water gels have a consistency resembling that of set gelatin or gel-type toothpaste. They are characterized by their water-resistant nature and are employed for all types of blasting under wet conditions. These explosives are based on formulations of ammonium nitrate and sodium nitrate gelled with a natural polysaccharide such as guar gum. Commonly, a combustible material such as aluminum is mixed into the gel to serve as the explosive’s fuel.

Emulsion explosives differ from gels in that they consist of two distinct phases, an oil phase and a water phase. In these emulsions, a droplet of a supersaturated solution of ammonium nitrate is surrounded by a hydrocarbon serving as a fuel. A typical emulsion consists of water, one or more inorganic nitrate oxidizers, oil, and emulsifying agents. Commonly, emulsions contain micron-sized glass, resin, or ceramic spheres known as microspheres or microballoons. The size of these spheres controls the explosive’s sensitivity and detonation velocity.

Ammonium nitrate soaked in fuel oil is an explosive known as ANFO. Such commercial explo- sives are inexpensive and safe to handle and have found wide applications in blasting operations in the mining industry. Ammonium nitrate in the form of fertilizer makes a readily obtainable ingredi- ent for homemade explosives. Indeed, in an incident related to the 1993 bombing of New York City’s World Trade Center, the FBI arrested five men during a raid on their hideout in New York City, where they were mixing a “witches’ brew” of fuel oil and an ammonium nitrate–based fertilizer.

TATP Triacetone triperoxide (TATP) is a homemade explosive that has been used as an impro- vised explosive by terrorist organizations in Israel and other Middle Eastern countries. It is prepared by reacting the common ingredients of acetone and hydrogen peroxide in the presence of an acid catalyst such as hydrochloric acid.

FIGURE 15–3 Blasting caps. The left and center caps are initiated by an electrical current; the right cap is initiated by a safety fuse.

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378 CHAPTER 15

TATP is a friction- and impact-sensitive explosive that is extremely potent when confined in a container such as a pipe. The 2005 London transit bombings were caused by TATP-based ex- plosives and provide ample evidence that terrorist cells have moved TATP outside the Middle East. A London bus destroyed by one of the TATP bombs is shown in Figure 15–5.

FIGURE 15–4 Sticks of dynamite. U.S. Department of Justice\AP Wide World Photos

FIGURE 15–5 A London bus destroyed by a TATP-based bomb. Courtesy AP Wide World Photos IS

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A plot to blow up ten international plane flights leaving Britain for the United States with a “liquid explosive” apparently involved plans to smuggle the peroxide-based TATP explosive onto the planes. This plot has prompted authorities to prohibit airline passengers from carrying liquids and gels onto planes.

Military High Explosives No discussion of high explosives would be complete without a men- tion of military high explosives. In many countries outside the United States, the accessibility of high explosives to terrorist organizations makes them common constituents of homemade bombs. RDX, the most popular and powerful military explosive, is often encountered in the form of a pliable plastic of doughlike consistency known as composition C–4 (a U.S. military designation).

TNT was produced and used on an enormous scale during World War II and may be consid- ered the most important military bursting charge explosive. Alone or in combination with other explosives, it has found wide application in shells, bombs, grenades, demolition explosives, and propellant compositions (see Figure 15–6). Interestingly, military “dynamite” contains no nitro- glycerin but is actually composed of a mixture of RDX and TNT. Like other military explosives, TNT is rarely encountered in bombings in the United States.

PETN is used by the military in TNT mixtures for small-caliber projectiles and grenades. Commercially, the chemical is used as the explosive core in a detonating cord or primacord. Instead of the slower-burning safety fuse, a detonating cord is often used to connect a series of explosive charges so that they will detonate simultaneously.

Detonators Unlike low explosives, bombs made of high explosives must be detonated by an ini- tiating explosion. In most cases, detonators are blasting caps composed of copper or aluminum cases filled with lead azide as an initiating charge and PETN or RDX as a detonating charge. Blasting caps can be initiated by means of a burning safety fuse or by an electrical current.

Homemade bombs camouflaged in packages, suitcases, and the like, are usually initiated with an electrical blasting cap wired to a battery. An unlimited number of switching-mechanism designs have been devised for setting off these devices; clocks and mercury switches are favored. Bombers sometimes prefer to employ outside electrical sources. For instance, most automobile bombs are detonated when the ignition switch of a car is turned on.

detonating cord A cordlike explosive containing a core of high-explosive material, usually PETN; also called primacord

FIGURE 15–6 Military explosives in combat use. Courtesy Getty Images/Time Life Pictures

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Liquid Explosives

In 2006, security agencies in the United States and Great Britain uncovered a terrorist plot to use liquid explosives to destroy commercial airlines operating between the two countries. Of the hundreds of types of explosives, most are solid. Only about a dozen are liquid. But some of those liquid explosives can be readily purchased and others can be made from hun- dreds of different kinds of chemicals that are not dif- ficult to obtain. After the September 11 attacks, worries about solid explosives became the primary concern. In 2001, Richard Reid was arrested for at- tempting to destroy an American Airlines flight out of Paris. Authorities later found a high explosive with a TATP detonator hidden in the lining of his shoe. It is therefore not surprising that terrorists turned to liq- uids in this latest plot. A memo issued by federal se- curity officials about the plot to blow up ten international planes highlighted a type of liquid ex- plosive based on peroxide.

forensics at work

Gels discarded by airline passengers before boarding. Stefano Paltera, AP Wide World Photos

380 CHAPTER 15

Collection and Analysis of Evidence of Explosives The most important step in the detection and analysis of explosive residues is the collection of appropriate samples from the explosion scene. Invariably, undetonated residues of the explosive remain at the site of the explosion. The detection and identification of these explosives in the lab- oratory depends on the bomb-scene investigator’s skill and ability to recognize and sample the areas most likely to contain such materials.

Detecting and Recovering Evidence of Explosives The most obvious characteristic of a high or contained low explosive is the presence of a crater at the origin of the blast. Once the crater has been located, all loose soil and other debris must immediately be removed from the interior of the hole and preserved for laboratory analysis. Other good sources of explosive residues are objects located near the origin of detonation. Wood, insula- tion, rubber, and other soft materials that are readily penetrated often collect traces of the explosive. However, nonporous objects near the blast must not be overlooked. For instance, residues can be

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forensics at work

The most common peroxide-based explosive is TATP (triacetone triperoxide), which is made up of acetone and hydrogen peroxide, two widely available substances. TATP can be used as a detonator or a pri- mary explosive and has been used in Qaeda-related bomb plots and by Palestinian suicide bombers. TATP itself is a white powder made up of crystals that form when acetone and hydrogen peroxide are mixed together, usually with a catalyst added to speed the chemical reactions. Acetone is the main ingredient in nail polish remover, while hydrogen peroxide is a pop- ular antiseptic. When the two main ingredients are mixed, they form a white powder that can be easily det- onated using an electrical spark.

Commercially available hydrogen peroxide, however, is not concentrated enough to create TATP. The solution sold in stores contains about 3 percent hydrogen per- oxide, compared to the approximately 70 percent con- centration need for TATP. However, hydrogen peroxide solutions of up to 30 percent can be obtained from

chemical supply houses. According to explosives experts, a mixture of 30 percent hydrogen peroxide and acetone can create a fire hot enough to burn though the fuselage of an aircraft.

In theory, scientists know how to detect peroxide- based explosives. The challenge is to design machines that can perform scans quickly and efficiently on thou- sands of passengers passing through airport security checks. Current scanning machines at airports are designed to detect nitrogen containing chemicals and are not designed to detect peroxide-containing explo- sive ingredients. Since 9/11, security experts have worried about the possibility of liquid explosives in the form of liquids and gels getting onto airliners.

Without the luxury of waiting for newly designed scanning devices capable of ferreting out dangerous liquids to be in place at airports, the decision was made to use a common-sense approach—that is, to restrict the types and quantities of liquids that a passenger can carry onto a plane.

found on the surfaces of metal objects near the site of an explosion. Material blown away from the blast’s origin should also be recovered because it, too, may retain explosive residues.

The entire area must be systematically searched, with great care given to recovering any trace of a detonating mechanism or any other item foreign to the explosion site. Wire-mesh screens are best used for sifting through debris. All personnel involved in searching the bomb scene must take appropriate measures to avoid contaminating the scene, including dressing in disposable gloves, shoe covers, and overalls.

ION MOBILITY SPECTROMETER In pipe-bomb explosions, particles of the explosive are frequently found adhering to the pipe cap or to the pipe threads, as a result of either being impacted into the metal by the force of the explosion or being deposited in the threads during the construction of the bomb. One approach for screening objects for the presence of explosive residues in the field or the labora- tory is the ion mobility spectrometer (IMS).1 A portable IMS is shown in Figure 15–7.

1 T. Keller et al., “Application of Ion Mobility Spectrometry in Cases of Forensic Interest,” Forensic Science International 161 (2006): 130.

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This handheld detector uses a vacuum to collect explosive residues from suspect surfaces. Alternatively, the surface suspected of containing explosive residues is wiped down with a Teflon- coated fiberglass disc and the collected residues are then drawn into the spectrometer off the disc. Once in the IMS, the explosive residues are vaporized by the application of heat. These vapor- ized substances are exposed to a beam of electrons or beta rays emitted by radioactive nickel and converted into electrically charged molecules or ions. The ions are then allowed to move through a tube (drift region) under the influence of an electric field. A schematic diagram of an IMS is shown in Figure 15–8.

The preliminary identification of an explosive residue can be made by noting the time it takes the explosive to move through the tube. Because ions move at different speeds depending on their size and structure, they can be characterized by the speed at which they pass through the tube. Used as a screening tool, this method rapidly detects a full range of explosives, even at low detection levels. However, all results need to be verified through confirmatory tests.

The IMS can detect plastic explosives as well as commercial and military explosives. More than 10,000 portable and full-size IMS units are currently used at airport security checkpoints, and more than 50,000 handheld IMS analyzers have been deployed for chemical-weapons monitoring in various armed forces.

COLLECTION AND PACKAGING All materials collected for examination by the laboratory must be placed in airtight sealed containers and labeled with all pertinent information. Soil and other soft loose materials are best stored in metal airtight containers such as clean paint cans. Debris and articles collected from different areas are to be packaged in separate airtight containers. Plas- tic bags should not be used to store evidence suspected of containing explosive residues. Some explosives can actually escape through the plastic. Sharp-edged objects should not be allowed to pierce the sides of a plastic bag. It is best to place these types of items in metal containers.

Analysis of Evidence of Explosives When the bomb-scene debris and other materials arrive at the laboratory, everything is first examined microscopically to detect particles of unconsumed explosive. Portions of the recovered debris and detonating mechanism, if found, are carefully viewed under a low-power stereoscopic microscope in a painstaking effort to locate particles of the explosive. Black powder and smoke- less powder are relatively easy to locate in debris because of their characteristic shapes and colors

382 CHAPTER 15

FIGURE 15–7 A portable ion mobility spectrometer used to rapidly detect and tentatively identify trace quantities of explosives. Courtesy GE Ion Track, Wilmington, Mass. 01887

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FIGURE 15–8 Schematic diagram of an ion mobility spectrometer. A sample is introduced into an ionization chamber, where bombardment with radioactive particles emitted by an isotope of nickel converts the sample to ions. The ions move into a drift region where ion separation occurs based on the speed of the ions as they move through an electric field.

Ionization chamber

Drift rings

Sample is bombarded by radioactive particles emitted by an isotope of nickel to form ionsSample is

drawn into ionization chamber

(a) Drift region

Collection electrode

Shutter

63Ni

Ionization chamber (b)

Drift rings Sample is converted into ions of different sizes and structures

Drift region

Collection electrode

Shutter

Ionization chamber

(c)

Drift rings

Explosive substances can be characterized by the speed at which they move through the electric field

Drift region Ions separate as they move through an electric field

Collection electrode

Shutter

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TABLE 15–1 Color Spot Tests for Common Explosives

Reagent

Substance Greissa Diphenylamineb Alcoholic KOHc

Chlorate No color Blue No color Nitrate Pink to red Blue No color Nitrocellulose Pink Blue-black No color Nitroglycerin Pink to red Blue No color PETN Pink to red Blue No color RDX Pink to red Blue No color TNT No color No color Red Tetryl Pink to red Blue Red-violet

a Greiss reagent: Solution 1—Dissolve 1 g sulfanilic acid in 100 mL 30% acetic acid. Solution 2—Dissolve 0.5 g N-(1-napthyl) ethylenediamine in 100 mL methyl alcohol. Add solutions 1 and 2 and a few milligrams of zinc dust to the suspect extract.

b Diphenylamine reagent: Dissolve 1 g diphenylamine in 100 mL concentrated sulfuric acid. c Alcoholic KOH reagent: Dissolve 10 g of potassium hydroxide in 100 mL absolute alcohol.

(see Figure 15–2). However, dynamite and other high explosives present the microscopist with a much more difficult task and often must be detected by other means.

Following microscopic examination, the recovered debris is thoroughly rinsed with acetone. The high solubility of most explosives in acetone ensures their quick removal from the debris. When a water-gel explosive containing ammonium nitrate or a low explosive is suspected, the debris should be rinsed with water so that water-soluble substances (such as nitrates and chlorates) will be extracted. Table 15–1 lists a number of simple color tests the examiner can perform on the acetone and water extracts to screen for the presence of organic and inorganic explosives, respectively.

SCREENING AND CONFIRMATION TESTS Once collected, the acetone extract is concentrated and analyzed using color spot tests, thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC; see pages 127–128), and gas chromatography/mass spectrometry. The presence of an explosive is indicated by a well-defined spot on a TLC plate with an Rf value corresponding to a known explosive—for example, nitroglycerin, RDX, or PETN.

The high sensitivity of HPLC also makes it useful for analyzing trace evidence of explosives. HPLC operates at room temperature and hence does not cause explosives, many of which are tem- perature sensitive, to decompose during their analysis. When a water-gel explosive containing ammonium nitrate or a low explosive is suspected, the debris should be rinsed with water so that water-soluble substances (such as nitrates and chlorates) will be extracted.

When sufficient quantities of explosives are recoverable, confirmatory tests may be performed by either infrared spectrophotometry or X-ray diffraction. The former produces a unique “fingerprint” pattern for an organic explosive, as shown by the IR spectrum of RDX in Figure 15–9. The latter provides a unique diffraction pattern for inorganic substances such as potassium nitrate and potassium chlorate, shown in Figure 6–11.

TAGGANTS An explosive “taggant” program has been proposed to further enhance a bomb- scene investigator’s chances of recovering useful evidence at a postexplosion scene. Under this proposal, tiny color-coded chips the size of sand grains would be added to commercial explosives during their manufacture. Some of these chips would be expected to survive an explosion and be capable of recovery at explosion scenes. To aid in their recovery, the chips are made both fluo- rescent and magnetic sensitive. Hence, investigators can search for taggants at the explosion site with magnetic tools and ultraviolet light.

The taggant chip is arranged in a color sequence that indicates where the explosive was made and when it was produced (see Figure 15–10). With this knowledge, the explosive can be traced through its distribution chain to its final legal possessor. The taggant colors are readily observed and are read with the aid of a low-power microscope.

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FORENSIC INVESTIGATION OF EXPLOSIONS 385

FIGURE 15–10 Cross-section of a taggant. The color sequence of the recovered taggant is observed with the aid of a low-power microscope. The colors are then matched to a color code to yield information about the plant of manufacture, production lot, and purchasers of the explosive material.

Fluorescent spotter

1 2 3 4 5 6 7 8

100

16151413121110987654

2000 1800 1600 1400 1200 1000 800 6252500300035003800

Wavenumber cm–1

Wavelength μm Tra

ns m

itt an

FIGURE 15–9 Infrared spectrum of RDX.

There are no plans to institute a taggant program for commercial explosives in the United States. In Europe, only Switzerland has adopted a taggant program; thus, it is extremely doubtful that taggants will be found in any significant number of bombing incidents in the foreseeable future. Interestingly, the International Civil Aviation Organization has mandated that a volatile taggant be added to plastic explosives during their manufacture in order to facilitate the detection of these explosives. Programs are now under way to tag commercial C–4 with the volatile chemical known as DMNB (2,3-dimethyl-2,3-dinitrobutane).

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386 CHAPTER 15

Explosives are substances that undergo a rapid oxidation reaction with the production of large quantities of gases. This sudden buildup of gas pressure constitutes an explosion. The speed at which explosives decompose permits their classifica- tion as high or low explosives.

The most widely used low explosives are black powder and smokeless powder. Among the high explosives, primary explosives are ultrasensitive to heat, shock, or friction and provide the major ingredients found in blasting caps. Sec- ondary explosives normally constitute the main charge of a high explosive.

Among the high explosives, nitroglycerin-based dynamite has all but disappeared from the industrial explosives market and has been replaced by ammonium nitrate–based explosives (such as water gels, emulsions, and ANFO explosives). In many countries outside the United States, the accessibility of

military high explosives to terrorist organizations makes them common constituents of homemade bombs. RDX is the most popular and powerful of the military explosives.

The entire bomb site must be systematically searched with great care given to recovering any trace of a detonating mechanism or any other item foreign to the explosion site. Objects located at or near the origin of the explosion must be collected for laboratory examination.

Typically, in the laboratory, debris collected at explosion scenes is examined microscopically for unconsumed explosive particles. Recovered debris may also be thoroughly rinsed with organic solvents and analyzed by testing procedures that include color spot tests, thin-layer chromatography, high- performance liquid chromatography, and gas chromatography/ mass spectrometry.

review questions

1. Oxidizing agents supply ___________ to a chemical reaction.

2. Three ingredients of black powder are ___________, ___________, and ___________.

3. Rapid combustion accompanied by the creation of large volumes of gases describes a(n) ___________.

4. Explosives that decompose at relatively slow rates are classified as ___________ explosives.

5. ___________ explosives detonate almost instanta- neously to produce a smashing or shattering effect.

6. The most widely used low explosives are ___________ and ___________.

7. A low explosive becomes explosive and lethal only when it is ___________.

8. True or False: Air and a gaseous fuel burn when mixed at any concentration range. ___________

9. High explosives can be classified as either ___________ or ___________ explosives.

10. The blasting power of different dynamite strengths (is, is not) in direct proportion to the weight percentage of nitroglycerin.

11. True or False: The most common commercial explosives incorporate ammonium nitrate. ___________

12. The most widely used explosive in the military is ___________.

13. The explosive core in detonating cord is ___________.

14. A high explosive is normally detonated by a(n) ___________ explosive contained within a blasting cap.

15. An obvious characteristic of a high explosive is the pres- ence of a(n) ___________ at the origin of the blast.

16. The most important step in detecting explosive residues is the ___________ of appropriate samples from the explosion scene.

17. To screen objects for the presence of explosive residues in the field or the laboratory, the investigator may use a handheld ___________.

18. Unconsumed explosive residues may be detected in the laboratory through a careful ___________ examination of the debris.

19. Debris recovered from the site of an explosion is rou- tinely rinsed with ___________ in an attempt to recover high-explosive residues.

20. Once collected, the acetone extract is initially analyzed by ___________, ___________, and ___________.

21. The technique of ___________ produces a unique absorption spectrum for an organic explosive.

> > > > > > > > > > >chapter summary

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FORENSIC INVESTIGATION OF EXPLOSIONS 387

22. The technique of ___________ provides a unique dif- fraction pattern for the identification of the inorganic constituents of explosives.

23. True or False: Debris and articles at an explosion scene that are collected from different areas are to be packaged in separate airtight containers.___________

24. True or False: In the absence of airtight metal containers, plastic bags can be used to store evidence suspected of containing explosive residues.___________

1. The following pieces of evidence were found at separate explosion sites. For each item, indicate whether the ex- plosion was more likely caused by low or high explo- sives, and explain your answer:

a. Lead azide residues

b. Nitrocellulose residues

c. Ammonium nitrate residues

d. Scraps of primacord

e. Potassium chlorate residues

2. Which color test or tests would you run first on a suspect sample to test for evidence of each of the following explosives? Explain your answers.

a. Tetryl

b. TNT

c. Chlorate

d. Nitrocellulose

3. Criminalist Matt Weir is collecting evidence from the site of an explosion. Arriving on the scene, he immedi- ately proceeds to look for the crater caused by the blast. After finding the crater, he picks through the debris at the site by hand, looking for evidence of detonators or for- eign materials. Matt collects loose soil and debris from the immediate area, placing the smaller bits in paper folded into a druggist fold. Larger items he stores in plas- tic bags for transportation to the laboratory. What mis- takes, if any, did Matt make in collecting and storing this evidence?

application and critical thinking

further references

Thurman, J. T., Practical Bomb Scene Investigation. Boca Raton, Fla.: Taylor & Francis, 2006.

Yinon, J., Forensic and Environmental Detection of Explo- sives. West Sussex, England: Wiley, 1999.

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James Earl Ray: Conspirator or Lone Gunman?

Since his arrest in 1968 for the assassination of Dr. Martin Luther King, Jr., endless speculation has swirled around the motives and connections of James Earl Ray. Ray was a career criminal who was serving time for armed robbery when he escaped from the Missouri State Prison almost one year before the assassination. On April 3, 1968, Ray arrived in Memphis, Tennessee. The next day he rented a room at Bessie Brewer’s Rooming House, across the street from the Lorraine Motel where Dr. King was staying.

At 6:00 p.m., Dr. King left his second-story motel room and stepped onto the balcony of the Lorraine Motel. As King turned toward his room, a shot rang out, striking the civil rights activist. Nothing could be done to revive him, and Dr. King was pronounced dead at 7:05 p.m. As the assailant ran on foot from Bessie Brewer’s, he left a blanket-covered package in front of a nearby building and then drove off in a white Mustang. The package was later shown to contain a high-powered rifle equipped with a scope, a radio, some clothes,

a pair of binoculars, a couple of beer cans, and a receipt for the binoculars. Almost a week after the shooting, the white

Mustang was found abandoned in Atlanta, Georgia. Fingerprints later identified as James Earl Ray’s were found in the Mustang, on the rifle,

on the binoculars, and on a beer can. In 1969, Ray entered a guilty plea in return for a sentence of ninety- nine years. Although a variety of conspiracy theories surround this crime, the indisputable fact is that a fingerprint put the rifle that killed Martin Luther King, Jr., in the hands of James Earl Ray.

headline news

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After studying this chapter you should be able to: • Know the common ridge characteristics of a fingerprint

• List the three major fingerprint patterns and their respective subclasses

• Distinguish visible, plastic, and latent fingerprints

• Describe the concept of an automated fingerprint identification system (AFIS)

• List the techniques for developing latent fingerprints on porous and nonporous objects

• Describe the proper procedures for preserving a developed latent fingerprint

fingerprints

anthropometry arch digital imaging fluoresce iodine fuming latent fingerprint livescan loop ninhydrin Physical Developer pixel plastic print portrait parlé ridge characteristics

(minutiae) sublimation superglue fuming visible print whorl

KEY TERMS

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portrait parlé A verbal description of a perpetrator’s physical characteristics and dress provided by an eyewitness

390 CHAPTER 16

History of Fingerprinting Since the beginnings of criminal investigation, police have sought an infallible means of human identification. The first systematic attempt at personal identification was devised and introduced by a French police expert, Alphonse Bertillon, in 1883. The Bertillon system relied on a detailed description (portrait parlé) of the subject, combined with full-length and profile photographs and a system of precise body measurements known as anthropometry.

The use of anthropometry as a method of identification rested on the premise that the di- mensions of the human bone system remained fixed from age 20 until death. Skeleton sizes were thought to be so extremely diverse that no two individuals could have exactly the same measure- ments. Bertillon recommended routine taking of eleven measurements of the human anatomy. These included height, reach, width of head, and length of the left foot (see Figure 1–2).

For two decades, this system was considered the most accurate method of identification. But in the first years of the new century, police began to appreciate and accept a system of identifica- tion based on the classification of finger ridge patterns known as fingerprints. Today, the finger- print is the pillar of modern criminal identification.

Early Use of Fingerprints Evidence exists that the Chinese used the fingerprint to sign legal documents as far back as three thousand years ago. However, whether this practice was performed for ceremonial custom or as a means of personal identity remains a point of conjecture lost to history. In any case, the exam- ples of fingerprinting in ancient history are ambiguous, and the few that exist did not contribute to the development of fingerprinting techniques as we know them today.

Several years before Bertillon began work on his system, William Herschel, an English civil servant stationed in India, started the practice of requiring Indian citizens to sign contracts with the im- print of their right hand, which was pressed against a stamp pad for the purpose. The motives for Herschel’s requirement remain unclear; he may have envisioned fingerprinting as a means of per- sonal identification or just as a form of the Hindu custom that a trace of bodily contact was more binding than a signature on a contract. In any case, he did not publish anything about his activi- ties until after a Scottish physician, Henry Fauld, working in a hospital in Japan, published his views on the potential application of fingerprinting to personal identification.

In 1880, Fauld suggested that skin ridge patterns could be important for the identification of criminals. He told about a thief who left his fingerprint on a whitewashed wall, and how in com- paring these prints with those of a suspect, he found that they were quite different. A few days later another suspect was found whose fingerprints compared with those on the wall. When con- fronted with this evidence, the individual confessed to the crime.

Fauld was convinced that fingerprints furnished infallible proof of identification. He even of- fered to set up, at his own expense, a fingerprint bureau at Scotland Yard to test the practicality of the method. But his offer was rejected in favor of the Bertillon system. This decision was reversed less than two decades later.

Early Classification of Fingerprints The extensive research into fingerprinting conducted by another Englishman, Francis Galton, provided the needed impetus that made police agencies aware of its potential application. In 1892, Galton published his classic textbook Finger Prints, the first book of its kind on the subject. In his book, he discussed the anatomy of fingerprints and suggested methods for recording them. Galton also proposed assigning fingerprints to three pattern types—loops, arches, and whorls. Most important, the book demonstrated that no two prints were identical and that an individual’s prints remained unchanged from year to year. At Galton’s insistence, the British government adopted fingerprinting as a supplement to the Bertillon system.

The next step in the development of fingerprint technology was the creation of classification systems capable of filing thousands of prints in a logical and searchable sequence. Dr. Juan Vucetich, an Argentinian police officer fascinated by Galton’s work, devised a workable concept in 1891. His classification system has been refined over the years and is still widely used today in most Spanish-speaking countries. In 1897, another classification system was proposed by an Englishman, Sir Edward Richard Henry. Four years later, Henry’s system was adopted by Scotland

anthropometry A system of identification of individuals by measurement of parts of the body, developed by Alphonse Bertillon

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ridge characteristics (minutiae) Ridge endings, bifurcations, enclosures, and other ridge details, which must match in two fingerprints in order for their common origin to be established

FINGERPRINTS 391

Yard. Today, most English-speaking countries, including the United States, use some version of Henry’s classification system to file fingerprints.

Adoption of Fingerprinting Early in the 20th century, Bertillon’s measurement system began to fall into disfavor. Its results were highly susceptible to error, particularly when the measurements were taken by people who were not thoroughly trained. The method was dealt its most severe and notable setback in 1903 when a convict, Will West, arrived at Fort Leavenworth prison. A routine check of the prison files startlingly revealed that a William West, already in the prison, could not be distinguished from the new prisoner by body measurements or even by photographs. In fact, the two men looked just like twins, and their measurements were practically the same. Subsequently, fingerprints of the pris- oners clearly distinguished them.

In the United States, the first systematic and official use of fingerprints for personal identifi- cation was adopted by the New York City Civil Service Commission in 1901. The method was used for certifying all civil service applications. Several American police officials received in- struction in fingerprint identification at the 1904 World’s Fair in St. Louis from representatives of Scotland Yard. After the fair and the Will West incident, fingerprinting began to be used in earnest in all major cities of the United States. In 1924, the fingerprint records of the Bureau of Investi- gation and Leavenworth were merged to form the nucleus of the identification records of the new Federal Bureau of Investigation. The FBI has the largest collection of fingerprints in the world. By the beginning of World War I, England and practically all of Europe had adopted fingerprint- ing as their primary method of identifying criminals.

In 1999, the admissibility of fingerprint evidence was challenged in the case of United States v. Byron C. Mitchell in the Eastern District of Pennsylvania. The defendant’s attorneys argued that fingerprints could not be proven unique under the guidelines cited in Daubert (see pages 16–17). Government experts vigorously disputed this claim. After a four-and-a-half-day Daubert hearing, the judge upheld the admissibility of fingerprints as scientific evidence and ruled that (1) human friction ridges are unique and permanent and (2) human friction ridge skin arrangements are unique and permanent.

Fundamental Principles of Fingerprints First Principle: A Fingerprint Is an Individual Characteristic; No Two Fingers Have Yet Been Found to Possess Identical Ridge Characteristics The acceptance of fingerprint evidence by the courts has always been predicated on the assump- tion that no two individuals have identical fingerprints. Early fingerprint experts consistently re- ferred to Galton’s calculation, showing the possible existence of 64 billion different fingerprints, to support this contention. Later, researchers questioned the validity of Galton’s figures and attempted to devise mathematical models to better approximate this value. However, no matter what mathematical model one refers to, the conclusions are always the same: the probability for the existence of two identical fingerprint patterns in the world’s population is extremely small.

Not only is this principle supported by theoretical calculations, but just as important, it is ver- ified by the millions of individuals who have had their prints classified during the past 110 years— no two have ever been found to be identical. The FBI has nearly 50 million fingerprint records in its computer database and has yet to find an identical image belonging to two different people.

RIDGE CHARACTERISTICS The individuality of a fingerprint is not determined by its general shape or pattern but by a careful study of its ridge characteristics (also known as minutiae). The identity, number, and relative location of characteristics such as those illustrated in Figure 16–1 impart individuality to a fingerprint. If two prints are to match, they must reveal characteristics that not only are identical but have the same relative location to one another in a print. In a judi- cial proceeding, a point-by-point comparison must be demonstrated by the expert, using charts similar to the one shown in Figure 16–2, in order to prove the identity of an individual.

If an expert were asked to compare the characteristics of the complete fingerprint, no diffi- culty would be encountered in completing such an assignment; the average fingerprint has as

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392 CHAPTER 16

Ridge

Endings

Bifurcation Ridge Ending

Enclosure

Ridge Island (Ridge Dot)

Bifurcation

FIGURE 16–1 Fingerprint ridge characteristics. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

many as 150 individual ridge characteristics. However, most prints recovered at crime scenes are partial impressions, showing only a segment of the entire print. Under these circumstances, the expert can compare only a small number of ridge characteristics from the recovered print to a known recorded print.

RIDGE COMPARISONS For years, experts have debated how many ridge comparisons are nec- essary to identify two fingerprints as the same. Numbers that range from 8 to 16 have been sug- gested as being sufficient to meet the criteria of individuality. However, the difficulty in establishing such a minimum is that no comprehensive statistical study has ever been undertaken to determine the frequency of occurrence of different ridge characteristics and their relative locations. Until such a study is undertaken and completed, no meaningful guidelines can be established for defining the uniqueness of a fingerprint.

FIGURE 16–2 A fingerprint exhibit illustrating the matching ridge characteristics between the crime- scene print and an inked impression of one of the suspect’s fingers.

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latent fingerprint A fingerprint made by the deposit of oils and/or perspiration; it is invisible to the naked eye

FINGERPRINTS 393

In 1973, the International Association for Identification, after a three-year study of this ques- tion, concluded that “no valid basis exists for requiring a predetermined minimum number of fric- tion ridge characteristics which must be present in two impressions in order to establish positive identification.” Hence, the final determination must be based on the experience and knowledge of the expert, with the understanding that others may profess honest differences of opinion on the uniqueness of a fingerprint if the question of minimal number of ridge characteristics exists. In 1995, members of the international fingerprint community at a conference in Israel issued the Ne’urim Declaration, which supported the 1973 International Association for Identification resolution.

Second Principle: A Fingerprint Remains Unchanged During an Individual’s Lifetime Fingerprints are a reproduction of friction skin ridges found on the palm side of the fingers and thumbs. Similar friction skin can also be found on the surface of the palms and soles of the feet. Apparently, these skin surfaces have been designed by nature to provide our bodies with a firmer grasp and a resistance to slippage. A visual inspection of friction skin reveals a series of lines cor- responding to hills (ridges) and valleys (grooves). The shape and form of the skin ridges are what one sees as the black lines of an inked fingerprint impression.

STRUCTURE OF THE SKIN Skin is composed of layers of cells. Those nearest the surface make up the outer portion of the skin known as the epidermis, and the inner skin is known as the dermis. A cross section of skin (see Figure 16–3) reveals a boundary of cells separating the epidermis and dermis. The shape of this boundary, made up of dermal papillae, determines the form and pattern of the ridges on the surface of the skin. Once the dermal papillae develop in the human fetus, the ridge patterns remain unchanged throughout life except to enlarge during growth.

Each skin ridge is populated by a single row of pores that are the openings for ducts leading from the sweat glands. Through these pores, perspiration is discharged and deposited on the sur- face of the skin. Once the finger touches a surface, perspiration, along with oils that may have been picked up by touching the hairy portions of the body, is transferred onto that surface, thereby leaving an impression of the finger’s ridge pattern (a fingerprint). Prints deposited in this manner are invisible to the eye and are commonly referred to as latent fingerprints.

CHANGING FINGERPRINTS Although it is impossible to change one’s fingerprints, there has been no lack of effort on the part of some criminals to obscure them. If an injury reaches deeply

Ridge island

Sweat pores

Epidermis

Papillae

Dermis

Duct of sweat gland

Sweat gland

Nerves of touch

FIGURE 16–3 Cross-section of human skin.IS

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arch A class of fingerprints characterized by ridge lines that enter the print from one side and flow out the other side

whorl A class of fingerprints that includes ridge patterns that are generally rounded or circular in shape and have two deltas

loop A class of fingerprints characterized by ridge lines that enter from one side of the pattern and curve around to exit from the same side of the pattern

394 CHAPTER 16

enough into the skin and damages the dermal papillae, a permanent scar will form. However, for this to happen, such a wound would have to penetrate 1 to 2 millimeters beneath the skin’s sur- face. Indeed, efforts at intentionally scarring the skin can only be self-defeating, for it would be totally impossible to obliterate all of the ridge characteristics on the hand, and the presence of permanent scars merely provides new characteristics for identification.

Perhaps the most publicized attempt at obliteration was that of the notorious gangster John Dillinger, who tried to destroy his own fingerprints by applying a corrosive acid to them. Prints taken at the morgue after he was shot to death, compared with fingerprints recorded at the time of a previous arrest, proved that his efforts had been fruitless (see Figure 16–4).

Third Principle: Fingerprints Have General Ridge Patterns That Permit Them to Be Systematically Classified All fingerprints are divided into three classes on the basis of their general pattern: loops, whorls, and arches. Sixty to 65 percent of the population have loops, 30 to 35 percent have whorls, and about 5 percent have arches. These three classes form the basis for all ten-finger classification systems presently in use.

LOOPS A typical loop pattern is illustrated in Figure 16–5. A loop must have one or more ridges entering from one side of the print, recurving, and exiting from the same side. If the loop opens toward the little finger, it is called an ulnar loop; if it opens toward the thumb, it is a radial loop. The pattern area of the loop is surrounded by two diverging ridges known as type lines. The ridge point at or nearest the type-line divergence and located at or directly in front of the point of divergence is known as the delta. To many, a fingerprint delta resembles the silt formation that builds up as a river flows into the entrance of a lake—hence, the analogy to the geological for- mation known as a delta. All loops must have one delta. The core, as the name suggests, is the approximate center of the pattern.

WHORLS Whorls are actually divided into four distinct groups, as shown in Figure 16–6: plain, central pocket loop, double loop, and accidental. All whorl patterns must have type lines and at least two deltas. A plain whorl and a central pocket loop have at least one ridge that makes a complete circuit. This ridge may be in the form of a spiral, oval, or any variant of a circle. If an imaginary line drawn between the two deltas contained within these two patterns touches any one of the spiral ridges, the pattern is a plain whorl. If no such ridge is touched, the pattern is a central pocket loop.

ARCHES Arches, the least common of the three general patterns, are subdivided into two dis- tinct groups: plain arches and tented arches, as shown in Figure 16–7. The plain arch is the sim- plest of all fingerprint patterns; it is formed by ridges entering from one side of the print and

FIGURE 16–4 The right index finger impression of John Dillinger, before scarification on the left and afterward on the right. Comparison is proved by the 14 matching ridge characteristics. Courtesy Institute of Applied Science, Youngsville, N.C.

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FINGERPRINTS 395

Type line

Delta

Type line

Core

FIGURE 16–5 Loop pattern.

exiting on the opposite side. Generally, these ridges tend to rise in the center of the pattern, form- ing a wavelike pattern. The tented arch is similar to the plain arch except that instead of rising smoothly at the center, there is a sharp upthrust or spike, or the ridges meet at an angle that is less than 90 degrees.1 Arches do not have type lines, deltas, or cores.

OTHER PATTERNS As the name implies, the double loop is made up of two loops combined into one fingerprint. Any whorl classified as an accidental either contains two or more patterns (not including the plain arch) or is a pattern not covered by other categories. Hence, an accidental may consist of a combination loop and plain whorl or loop and tented arch.

With a knowledge of basic fingerprint pattern classes, we can now begin to develop an ap- preciation for fingerprint classification systems. However, the subject is far more complex than can be described in a textbook of this nature. The student seeking a more detailed treatment of the subject would do well to consult the references cited at the end of the chapter.

Classification of Fingerprints The original Henry system, as it was adopted by Scotland Yard in 1901, converted ridge patterns on all ten fingers into a series of letters and numbers arranged in the form of a fraction. However, the system as it was originally designed could accommodate files of up to only 100,000 sets of

Plain whorl Central pocket loop

Double loop Accidental

FIGURE 16–6 Whorl patterns.

Plain Tented

FIGURE 16–7 Arch patterns.

1 A tented arch is also any pattern that resembles a loop but lacks one of the essential requirements for classification as a loop.

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396 CHAPTER 16

prints; thus, as collections grew in size, it became necessary to expand the capacity of the classi- fication system. In the United States, the FBI, faced with the problem of filing ever-increasing numbers of prints, expanded its classification capacity by modifying and extending the original Henry system. These modifications are collectively known as the FBI system and are used by most agencies in the United States today.

The Primary Classification Although we will not discuss all of the different divisions of the FBI system, a description of just one part, the primary classification, will provide an interesting insight into the process of finger- print classification.

The primary classification is part of the original Henry system and provides the first classi- fication step in the FBI system. Using this classification alone, all of the fingerprint cards in the world could be divided into 1,024 groups. The first step in obtaining the primary classification is to pair up fingers, placing one finger in the numerator of a fraction, the other in the denominator. The fingers are paired in the following sequence:

R. Index R. Ring L. Thumb L. Middle L. Little

R. Thumb R. Middle R. Little L. Index L. Ring

The presence or absence of the whorl pattern is the basis for determination of the primary classification. If a whorl pattern is found on any finger of the first pair, it is assigned a value of 16; on the second pair, a value of 8; on the third pair, a value of 4; on the fourth pair, a value of 2; and on the last pair, a value of 1. Any finger with an arch or loop pattern is assigned a value of 0.

After values for all ten fingers are obtained in this manner, they are totaled, and 1 is added to both the numerator and denominator. The fraction thus obtained is the primary classification. For example, if the right index and right middle fingers are whorls and all the others are loops, the primary classification is

16 � 0 � 0 � 0 � 0 � 1

0 � 8 � 0 � 0 � 0 � 1

Approximately 25 percent of the population falls into the 1/1 category; that is, all their fingers have either loops or arches.

A fingerprint classification system cannot in itself unequivocally identify an individual; it merely provides the fingerprint examiner with a number of candidates, all of whom have an indistinguishable set of prints in the system’s file. The identification must always be made by a final visual comparison of the suspect print’s and file print’s ridge characteristics; only these features can impart individuality to a fingerprint. Although ridge patterns impart class character- istics to the print, the type and position of ridge characteristics give it its individual character.

Automated Fingerprint Identification Systems The Henry system and its subclassifications have proven to be a cumbersome system for storing, retrieving, and searching for fingerprints, particularly as fingerprint collections grow in size. Nevertheless, until the emergence of fingerprint computer technology, this manual approach was the only viable method for the maintenance of fingerprint collections. Since 1970, technological advances have made possible the classification and retrieval of fingerprints by computers. Automated Fingerprint Identification Systems (AFISs) have proliferated throughout the law enforcement community.

In 1999, the FBI initiated full operation of the Integrated Automated Fingerprint Identifica- tion System (IAFIS), the largest AFIS in the United States, which links state AFIS computers with the FBI database. This database contains nearly 50 million fingerprint records. However, an AFIS can come in all sizes ranging from the FBI’s to independent systems operated by cities, counties, and other agencies of local government (see Figure 16–8). Unfortunately, these local systems often are not linked to the state’s AFIS system because of differences in software configurations.

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How AFIS Works The heart of AFIS technology is the ability of a computer to scan and digitally encode fingerprints so that they can be subject to high-speed computer processing. The AFIS uses automatic scan- ning devices that convert the image of a fingerprint into digital minutiae that contain data showing ridges at their points of termination (ridge endings) and the branching of ridges into two ridges (bifurcations). The relative position and orientation of the minutiae are also determined, allowing the computer to store each fingerprint in the form of a digitally recorded geometric pattern.

The computer’s search algorithm determines the degree of correlation between the location and relationship of the minutiae for both the search and file prints. In this manner, a computer can make thousands of fingerprint comparisons in a second; for example, a set of ten fingerprints can be searched against a file of 500,000 ten-finger prints (ten-prints) in about eight-tenths of a sec- ond. During the search for a match, the computer uses a scoring system that assigns prints to each of the criteria set by an operator. When the search is complete, the computer produces a list of file prints that have the closest correlation to the search prints. All of the selected prints are then examined by a trained fingerprint expert, who makes the final verification of the print’s identity. Thus, the AFIS makes no final decisions on the identity of a fingerprint, leaving this function to the eyes of a trained examiner.

The speed and accuracy of ten-print processing by AFIS have made possible the search of single latent crime-scene fingerprints against an entire file’s print collection. Before the AFIS, po- lice were usually restricted to comparing crime-scene fingerprints against those of known sus- pects. The impact of the AFIS on no-suspect cases has been dramatic. Minutes after California’s AFIS network received its first assignment, the computer scored a direct hit by identifying an in- dividual who had committed 15 murders, terrorizing the city of Los Angeles. Police estimate that it would have taken a single technician, manually searching the city’s 1.7 million print cards,

FIGURE 16–8 An AFIS system designed for use by local law enforcement agencies. Courtesy AFIX Technologies Inc., Pittsburg, KS 66762, www.afix.net

FIGURE 16–9 A side-by-side comparison of a latent print against a file fingerprint is conducted in seconds, and their similarity rating (SIM) is displayed on the upper-left portion of the screen. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.comIS

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livescan An inkless device that captures the digital images of fingerprints and palm prints and electronically transmits the images to an AFIS

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67 years to come up with the perpetrator’s prints. With the AFIS, the search took approximately 20 minutes. In its first year of operation, San Francisco’s AFIS computer conducted 5,514 latent fingerprint searches and achieved 1,001 identifications—a hit rate of 18 percent. This compares to the previous year’s average of 8 percent for manual latent-print searches.

As an example of how an AFIS computer operates, one system has been designed to auto- matically filter out imperfections in a latent print, enhance its image, and create a graphic repre- sentation of the fingerprint’s ridge endings and bifurcations and their direction. The print is then computer searched against file prints. The image of the latent print and a matching file print are then displayed side by side on a high-resolution video monitor, as shown in Figure 16–9. The matching latent and file prints are then verified and charted by a fingerprint examiner at a video workstation.

The stereotypical image of a booking officer rolling inked fingers onto a standard ten-print card for ultimate transmission to a database has, for the most part, been replaced with digital-capture devices (livescan) that eliminate ink and paper. The livescan captures the image on each finger and the palms as they are lightly pressed against a glass platen. These livescan images can then be sent to the AFIS database electronically, so that within minutes the booking agency can enter the fingerprint record into the AFIS database and search the database for previous entries of the same individual. See Figure 16–10.

FIGURE 16–10 Livescan technology enables law enforcement to print and compare a subject’s fingerprints rapidly, without inking the fingerprints. Courtesy MorphoTrak, Inc.

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Considerations with AFIS AFIS has fundamentally changed the way criminal investigators operate, allowing them to spend less time developing suspect lists and more time investigating the suspects generated by the com- puter. However, investigators must be cautioned against overreliance on a computer. Sometimes a latent print does not make a hit because of the poor quality of the file print. To avoid these potential problems, investigators must still print all known suspects in a case and manually search these prints against the crime-scene prints.

AFIS computers are available from several different suppliers. Each system scans finger- print images and detects and records information about minutiae (ridge endings and bifurcations); however, they do not all incorporate exactly the same features, coordinate systems, or units of measure to record fingerprint information. These software incompatibilities often mean that, although state systems can communicate with the FBI’s IAFIS, they may not communicate with each other directly. Likewise, local and state systems frequently cannot share information with each other. Many of these technical problems will be resolved as more agencies follow transmission standards developed by the National Institute of Standards and Technology and the FBI.

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> > > > > > > > > > > > > > > > > The Night Stalker

Richard Ramirez committed his first murder in June 1984. His victim was a 79-year-old woman who was stabbed repeatedly and sexually assaulted and then had her throat slashed. It would be eight months before Ramirez murdered again. In the spring, Ramirez began a murderous rampage that resulted in 13 additional killings and 5 rapes.

His modus operandi was to enter a home through an open window, shoot the male resi- dents, and savagely rape his female victims. He scribed a pentagram on the wall of one of his vic- tims and the words Jack the Knife, and was re- ported by another to force her to “swear to Satan” during the assault. His identity still unknown, the news media dubbed him the “Night Stalker.” As the body count continued to rise, public hysteria and a media frenzy prevailed.

The break in the case came when the license plate of what seemed to be a suspicious car related to a sighting of the Night Stalker was reported to the police. The police determined that the car had been stolen and eventually located it, abandoned in a parking lot. After processing the car for prints, police found one usable partial fingerprint. This fin- gerprint was entered into the Los Angeles Police Department’s brand-new AFIS computerized fin- gerprint system.

The Night Stalker was identified as Richard Ramirez, who had been fingerprinted following a traffic violation some years before. Police searching the home of one of his friends found the gun used

to commit the murders, and jewelry belonging to his victims was found in the possession of Ramirez’s sister. Ramirez was convicted of murder and sen- tenced to death in 1989. He remains on death row.

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The Mayfield Affair

On March 11, 2004, a series of ten explosions at four sites occurred on commuter trains traveling to or near the Atocha train station in Madrid, Spain. The death toll from these explosions was nearly 200, with more than 1,500 injured. On the day of the attack, a plastic bag was found in a van previously reported as stolen. The bag contained copper detonators like those used on the train bombs. On March 17, the FBI received electronic images of latent fingerprints that were recovered from the plastic bag. A search was initiated on the FBI’s IAFIS. A senior fingerprint examiner encoded seven minutiae points from the high-resolution image of one suspect la- tent fingerprint and initiated an IAFIS search matching the print to Brandon Mayfield.

Mayfield’s prints were in the FBI’s central database because they had been taken when he joined the military, where he served for eight years before being honorably discharged as a second lieutenant. After a visual compar- ison of the suspect and file prints, the examiner con- cluded a “100 percent match.” The identification was verified by a retired FBI fingerprint examiner with more than 30 years of experience who was under contract with the bureau, as well as by a court-appointed independent fingerprint examiner (see Figure 16–11).

Mayfield, age 37, a Muslim convert, was arrested on May 6 on a material witness warrant. The U.S. Attorney’s Office came up with a list of Mayfield’s potential ties to Muslim terrorists, which they included in the affidavit they presented to the federal judge who ordered his arrest and detention. The document also said that, although no travel records were found for Mayfield, “It is believed that Mayfield may have traveled under a false or fictitious name.” On May 24, after the Spaniards had linked the print from the plastic bag to an Algerian national, Mayfield’s case was thrown out. The FBI issued him a highly unusual official apology, and his ordeal became a stunning embarrassment to the U.S. government.

As part of its corrective-action process, the FBI formed an international committee of distinguished

forensics at work

latent-print examiners and forensic experts. Their task was to review the analysis performed by the FBI Labo- ratory and make recommendations that would help prevent this type of error in the future. The committee came up with some startling findings and observations (available at www.fbi.gov/hq/lab/fsc/backissu/jan2005/ special_report/2005_special_report.htm).

The committee members agreed that “the quality of the images that were used to make the erroneous identification was not a factor. . . . [T]he identification is filled with dissimilarities that were easily observed when a detailed analysis of the latent print was conducted.”

They further stated,

the power of the IAFIS match, coupled with the inherent pressure of working an extremely high-profile case, was thought to have influenced the initial examiner’s judg- ment and subsequent examination. . . . The apparent mindset of the initial examiner after reviewing the results of the IAFIS search was that a match did exist; therefore, it would be reasonable to assume that the other charac- teristics must match as well. In the absence of a detailed analysis of the print, it can be a short distance from find- ing only seven characteristics sufficient for plotting, prior to the automated search, to the position of 12 or 13 matching characteristics once the mind-set of identi- fication has become dominant. . . .

Once the mind-set occurred with the initial examiner, the subsequent examinations were tainted. . . . because of the inherent pressure of such a high- profile case, the power of an IAFIS match in conjunc- tion with the similarities in the candidate’s print, and the knowledge of the previous examiners’ conclu- sions (especially since the initial examiner was a highly respected supervisor with many years of expe- rience), it was concluded that subsequent examina- tions were incomplete and inaccurate. To disagree was not an expected response. . . . when the individ- ualization had been made by the examiner, it

Methods of Detecting Fingerprints Through common usage, the term latent fingerprint has come to be associated with any finger- print discovered at a crime scene. Sometimes, however, prints found at the scene of a crime are quite visible to the eye, and the word latent is a misnomer. Actually, there are three kinds of crime- scene prints: visible prints are made by fingers touching a surface after the ridges have been in contact with a colored material such as blood, paint, grease, or ink; plastic prints are ridge im- pressions left on a soft material such as putty, wax, soap, or dust; and latent or invisible prints are impressions caused by the transfer of body perspiration or oils present on finger ridges to the surface of an object.

plastic print A fingerprint impressed in a soft surface

visible print A fingerprint made when the finger deposits a visible material such as ink, dirt, or blood onto a surface

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decision to undertake research to develop more objec- tive standards for fingerprint identification.

An internal review of the FBI Latent Print Unit conducted in the aftermath of the Mayfield affair has resulted in the implementation of revisions in training, as well as in the decision-making process when deter- mining the comparative value of a latent print, along with more stringent verification policies and procedures (M. A. Smrz et al., Journal of Forensic Identification 56 [2006]: 402–34).

The impact of the Mayfield affair on fingerprint technology as currently practiced and the weight courts will assign to fingerprint matches remain open questions.

FIGURE 16–11 (a) Questioned print recovered in connection with the Madrid bombing investigation. (b) File print of Brandon Mayfield. (a) Courtesy Sirchie Fingerprint Laboratories, Youngsville, NC, www.sirchie.com, U.S. Department of Justice

(a) (b)

became increasingly difficult for others in the agency to disagree.

The committee went on to make a number of quality- assurance recommendations to help avoid a recur- rence of this type of error.

The Mayfield incident has also been the subject of an investigation by the Office of the Inspector General (OIG), U.S. Department of Justice (www.usdoj.gov/ oig/special/s0601/final.pdf). The OIG investigation concluded that a “series of systemic issues” in the FBI Laboratory contributed to the Mayfield misidentifica- tion. The report noted that the FBI has made signifi- cant procedural modifications to help prevent similar errors in the future and strongly supported the FBI’s

Locating Fingerprints Locating visible or plastic prints at the crime scene normally presents little problem to the inves- tigator because these prints are usually distinct and visible to the eye. Locating latent or invisible prints is obviously much more difficult and requires the use of techniques to make the print visi- ble. Although the investigator can choose from several methods for visualizing a latent print, the choice depends on the type of surface being examined.

Hard and nonabsorbent surfaces (such as glass, mirror, tile, and painted wood) require different development procedures from surfaces that are soft and porous (such as papers, card- board, and cloth). Prints on the former are preferably developed by the application of a powder

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or treatment with superglue, whereas prints on the latter generally require treatment with one or more chemicals.

Sometimes the most difficult aspect of fingerprint examination is the location of prints. Re- cent advances in fingerprint technology have led to the development of an ultraviolet image con- verter for the purpose of detecting latent fingerprints. This device, called the Reflected Ultraviolet Imaging System (RUVIS), can locate prints on most nonabsorbent surfaces without the aid of chemical or powder treatments (see Figure 16–12).

RUVIS detects the print in its natural state by aiming UV light at the surface suspected of containing prints. When the UV light strikes the fingerprint, the light is reflected back to the viewer, differentiating the print from its background surface. The transmitted UV light is then converted into visible light by an image intensifier. Once the print is located in this manner, the crime-scene investigator can develop it in the most appropriate fashion. See Figure 16–13.

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FIGURE 16–12 A Reflected Ultraviolet Imaging System allows an investigator to directly view surfaces for the presence of untreated latent fingerprints. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

FIGURE 16–13 Using a Reflected Ultraviolet Imaging System with the aid of a UV lamp to search for latent fingerprints. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

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Developing Latent Prints

FINGERPRINT POWDERS Fingerprint powders are commercially available in a variety of com- positions and colors. These powders, when applied lightly to a nonabsorbent surface with a camel’s-hair or fiberglass brush, readily adhere to perspiration residues and/or deposits of body oils left on the surface (see Figure 16–14).

Experienced examiners find that gray and black powders are adequate for most latent-print work; the examiner selects the powder that affords the best color contrast with the surface being dusted. Hence, the gray powder, composed of an aluminum dust, is used on dark-colored surfaces. It is also applied to mirrors and metal surfaces that are polished to a mirrorlike finish because these surfaces photograph as black. The black powder, composed basically of black carbon or charcoal, is applied to white or light-colored surfaces.

Other types of powders are available for developing latent prints. A magnetic-sensitive pow- der can be spread over a surface with a magnet in the form of a Magna Brush. A Magna Brush does not have any bristles to come in contact with the surface, so there is less chance that the print will be destroyed or damaged. The magnetic-sensitive powder comes in black and gray and is es- pecially useful on such items as finished leather and rough plastics, where the minute texture of the surface tends to hold particles of ordinary powder. Fluorescent powders are also used to de- velop latent fingerprints. These powders fluoresce under ultraviolet light. By photographing the fluorescence pattern of the developing print under UV light, it is possible to avoid having the color of the surface obscure the print.

IODINE FUMING Of the several chemical methods used for visualizing latent prints, iodine fuming is the oldest. Iodine is a solid crystal that, when heated, is transformed into a vapor with- out passing through a liquid phase; such a transformation is called sublimation. Most often, the suspect material is placed in an enclosed cabinet along with iodine crystals (see Figure 16–15). As the crystals are heated, the resultant vapors fill the chamber and combine with constituents of the latent print to make it visible. The reasons why latent prints are visualized by iodine vapors are not yet fully understood. Many believe that the iodine fumes combine with fatty oils; how- ever, there is also convincing evidence that the iodine may actually interact with residual water left on a print from perspiration.2

Unfortunately, iodine prints are not permanent and begin to fade once the fuming process is stopped. Therefore, the examiner must photograph the prints immediately on development in order to retain a permanent record. Also, iodine-developed prints can be fixed with a 1 percent solution of starch in water, applied by spraying. The print turns blue and lasts for several weeks to several months.

FINGERPRINTS 403

2 J. Almag, Y. Sasson, and A. Anati, “Chemical Reagents for the Development of Latent Fingerprints II: Controlled Addi- tion of Water Vapor to Iodine Fumes—A Solution to the Aging Problem,” Journal of Forensic Sciences 24 (1979): 431.

FIGURE 16–14 Developing a latent fingerprint on a surface by applying a fingerprint powder with a fiberglass brush. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

iodine fuming A technique for visualizing latent fingerprints by exposing them to iodine vapors

sublimation A physical change from the solid directly into the gaseous state

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superglue fuming A technique for visualizing latent fingerprints on nonporous surfaces by exposing them to cyanoacrylate vapors; named for the commercial product Super Glue.

Physical Developer A silver nitrate–based reagent formulated to develop latent fingerprints on porous surfaces

ninhydrin A chemical reagent used to develop latent fingerprints on porous materials by reacting with amino acids in perspiration

404 CHAPTER 16

FIGURE 16–15 A heated fuming cabinet. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

3 F. G. Kendall and B. W. Rehn, “Rapid Method of Superglue Fuming Application for the Development of Latent Fingerprints,” Journal of Forensic Sciences 28 (1983): 777.

NINHYDRIN Another chemical used for visualizing latent prints is ninhydrin. The development of latent prints with ninhydrin depends on its chemical reaction to form a purple-blue color with amino acids present in trace amounts in perspiration. Ninhydrin (triketohydrindene hydrate) is commonly sprayed onto the porous surface from an aerosol can. A solution is prepared by mix- ing the ninhydrin powder with a suitable solvent, such as acetone or ethyl alcohol; a 0.6 percent solution appears to be effective for most applications.

Generally, prints begin to appear within an hour or two after ninhydrin application; however, weaker prints may be visualized after 24 to 48 hours. The development can be hastened if the treated specimen is heated in an oven or on a hot plate at a temperature of 80–100°C. The ninhy- drin method has developed latent prints on paper as old as 15 years.

PHYSICAL DEVELOPER Physical Developer is a third chemical mixture used for visualizing latent prints. Physical Developer is a silver nitrate–based liquid reagent. The procedure for prepar- ing and using Physical Developer is described in Appendix IV. This method has gained wide acceptance by fingerprint examiners, who have found it effective for visualizing latent prints that remain undetected by the previously described methods. Also, this technique is effective for developing latent fingerprints on porous articles that may have been wet at one time.

For most fingerprint examiners, the chemical method of choice is ninhydrin. Its extreme sensitivity and ease of application have all but eliminated the use of iodine for latent-print visualization. However, when ninhydrin fails, development with Physical Developer may provide identifiable results. Application of Physical Developer washes away any traces of proteins from an object’s surface; hence, if one wishes to use all of the previously mentioned chemical development methods on the same surface, it is necessary to first fume with iodine, follow this treatment with ninhydrin, and then apply Physical Developer to the object.

SUPERGLUE FUMING In the past, chemical treatment for fingerprint development was reserved for porous surfaces such as paper and cardboard. However, since 1982, a chemical technique known as superglue fuming has gained wide popularity for developing latent prints on non- porous surfaces such as metals, electrical tape, leather, and plastic bags.3 See Figure 16–16.

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Superglue is approximately 98–99 percent cyanoacrylate ester, a chemical that interacts with and visualizes a latent fingerprint. Cyanoacrylate ester fumes can be created when superglue is placed on absorbent cotton treated with sodium hydroxide. The fumes can also be created by heat- ing the glue. The fumes and the evidential object are contained within an enclosed chamber for up to six hours. Development occurs when fumes from the glue adhere to the latent print, usually producing a white-appearing latent print. Interestingly, small enclosed areas, such as the interior of an automobile, have been successfully processed for latent prints with fumes from superglue.

Through the use of a small handheld wand, cyanoacrylate fuming is now easily done at a crime scene or in a laboratory setting. The wand heats a small cartridge containing cyanoacrylate. Once heated, the cyanoacrylate vaporizes, allowing the operator to direct the fumes onto the suspect area (see Figure 16–17).

FIGURE 16–16 Superglue fuming a nonporous metallic surface in the search for latent fingerprints. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

(a)

FIGURE 16–17 (a) A handheld fuming wand uses disposable cartridges containing cyanoacrylate. The wand is used to develop prints at the crime scene and (b) in the laboratory. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

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OTHER TECHNIQUES FOR VISUALIZATION One of the most exciting and dynamic areas of re- search in forensic science today is the application of chemical techniques to the visualization of latent fingerprints. Changes are occurring rapidly as researchers uncover a variety of processes applicable to the visualization of latent fingerprints. Interestingly, for many years progress in this field was minimal, and fingerprint specialists traditionally relied on three chemical techniques— iodine, ninhydrin, and silver nitrate—to reveal a hidden fingerprint. Then superglue fuming ex- tended chemical development to prints deposited on nonporous surfaces.

Fluorescence The first hint of things to come was the discovery that latent fingerprints could be visualized by exposure to laser light. This laser method took advantage of the fact that perspira- tion contains a variety of components that fluoresce when illuminated by laser light. Fluorescence occurs when a substance absorbs light and reemits the light in wavelengths longer than the illu- minating source. Importantly, substances that emit light or fluoresce are more readily seen with either the naked eye or through photography than are non-light-emitting materials. The high sen- sitivity of fluorescence serves as the underlying principle of many of the new chemical techniques used to visualize latent fingerprints.

The earliest use of fluorescence to visualize fingerprints came with the direct illumination of a fingerprint with argon–ion lasers. This laser type was chosen because its blue-green light out- put induced some of the perspiration components of a fingerprint to fluoresce (see Figure 16–18). The major drawback of this approach is that the perspiration components of a fingerprint are of- ten present in quantities too minute to observe even with the aid of fluorescence. The fingerprint examiner, wearing safety goggles containing optical filters, visually examines the specimen be- ing exposed to the laser light. The filters absorb the laser light and permit the wavelengths at which latent-print residues fluoresce to pass through to the eyes of the wearer. The filter also pro- tects the operator against eye damage from scattered or reflected laser light. Likewise, latent-print residue producing sufficient fluorescence can be photographed by placing this same filter across the lens of the camera. Examination of specimens and photography of the fluorescing latent prints are carried out in a darkened room.

Chemically Induced Fluorescence The next advancement in latent-fingerprint development occurred with the discovery that fingerprints could be treated with chemicals that would induce fluorescence when exposed to laser illumination. For example, the application of zinc chloride after ninhydrin treatment or the application of the dye rhodamine 6G after superglue fuming caused fluorescence and increased the sensitivity of detection on exposure to laser illumination. The dis- covery of numerous chemical developers for visualizing fingerprints through fluorescence quickly followed. This knowledge set the stage for the next advance in latent-fingerprint development— the alternate light source.

With the advent of chemically induced fluorescence, lasers were no longer needed to induce fingerprints to fluoresce through their perspiration residues. High-intensity light sources or alter- nate light sources have proliferated and all but replaced laser lights. See Figure 16–19. High- intensity quartz halogen or xenon-arc light sources can be focused on a suspect area through a

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Directional mirror Laser

Dispersal lens

Barrier filter Observer

FIGURE 16–18 Schematic depicting latent-print detection with the aid of a laser. A fingerprint examiner, wearing safety goggles containing optical filters, examines the specimen being exposed to the laser light. The filter absorbs the laser light and permits the wavelengths at which latent-print residues fluoresce to pass through to the eyes of the wearer. Courtesy Federal Bureau of Investigation, Washington, D.C.

fluoresce To emit visible light when exposed to light of a shorter wavelength

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FINGERPRINTS 407

fiber-optic cable. This light can be passed through several filters, giving the user more flexibility in selecting the wavelength of light to be aimed at the latent print. Alternatively, lightweight, portable alternate light sources that use light-emitting diodes (LEDs) are also commercially avail- able (see Figure 16–20). In most cases, these light sources have proven to be as effective as laser light in developing latent prints, and they are commercially available at costs significantly less than those of laser illuminators. Furthermore, these light sources are portable and can be readily taken to any crime scene.

NEWER CHEMICAL PROCESSES A large number of chemical treatment processes are available to the fingerprint examiner (see Figure 16–21), and the field is in a constant state of flux. Selec- tion of an appropriate procedure is best left to technicians who have developed their skills through casework experience.

Newer chemical processes include a substitute for ninhydrin called DFO (1,8-diazafluoren- 9-one). This chemical visualizes latent prints on porous materials when exposed to an alternate light source. DFO has been shown to develop 2.5 times more latent prints on paper than ninhy- drin. A chemical called 1,2-indanedione is also emerging as a potential reagent for the develop- ment of latent fingerprints on porous surfaces. 1,2-indanedione gives both good initial color and

FIGURE 16–19 An alternate light source system incorporating a high-intensity light source. Courtesy Foster & Freeman Limited, Worcestershire, U.K., www.fosterfreeman.co.uk

FIGURE 16–20 Lightweight handheld alternate light source that uses an LED light source. Courtesy Foster & Freeman Limited, Worcestershire, U.K., www.fosterfreeman.co.uk

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FIGURE 16–21 (a) Latent fingerprint visualized by cyanoacrylate fuming. (b) Fingerprint treated with cyanoacrylate and rhodamine 6G fluorescent dye. (c) Fingerprint treated with cyanoacrylate and the fluorescent dye combination RAM. (d) A bloody fingerprint detected by laser light after spraying with merbromin and hydrogen peroxide. Courtesy North Carolina State Bureau of Investigation, Raleigh, N.C.

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strong fluorescence when reacted with amino acids derived from prints and thus has the potential to provide in one process what ninhydrin and DFO can do in two different steps.

Dye combinations known as RAM, RAY, and MRM 10, when used in conjunction with su- perglue fuming, have been effective in visualizing latent fingerprints by fluorescence. A number of chemical formulas useful for latent-print development are listed in Appendix IV.

Studies have demonstrated that common fingerprint-developing agents do not interfere with DNA-testing methods used for characterizing bloodstains.4 Nonetheless, in cases involving items with material adhering to their surfaces and/or items that will require further laboratory exami- nations, fingerprint processing should not be performed at the crime scene. Rather, the items should be submitted to the laboratory, where they can be processed for fingerprints in conjunc- tion with other necessary examinations.

Preservation of Developed Prints Once the latent print has been visualized, it must be permanently preserved for future compari- son and possible use in court as evidence. A photograph must be taken before any further attempts at preservation. Any camera equipped with a close-up lens will do; however, many investigators prefer to use a camera specially designed for fingerprint photography. Such a camera comes equipped with a fixed focus to take photographs on a 1:1 scale when the camera’s open eye is held exactly flush against the print’s surface (see Figure 16–22). In addition, photographs must be taken to provide an overall view of the print’s location with respect to other evidential items at the crime scene.

4 C. Roux et al., “A Further Study to Investigate the Effect of Fingerprint Enhancement Techniques on the DNA Analysis of Bloodstains,” Journal of Forensic Identification 49 (1999): 357; C. J. Frégeau et al., “Fingerprint Enhancement Revisited and the Effects of Blood Enhancement Chemicals on Subsequent Profiler Plus™ Fluorescent Short Tandem Repeat DNA Analysis of Fresh and Aged Bloody Fingerprints,” Journal of Forensic Sciences 45 (2000): 354; and P. Grubwieser et al., “Systematic Study on STR Profiling on Blood and Saliva Traces after Visualization of Fingerprints,” Journal of Forensic Sciences 48 (2003): 733.

FIGURE 16–22 Camera fitted with an adapter designed to give an approximate 1:1 photograph of a fingerprint. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

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pixel A square electronic dot that is used to compose a digital image

digital imaging A process through which a picture is converted into a series of square electronic dots known as pixels; the picture is manipulated by computer software that changes the numerical value of each pixel

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Once photographs have been secured, one of two procedures is to be followed. If the object is small enough to be transported without destroying the print, it should be preserved in its en- tirety; the print should be covered with cellophane so it will be protected from damage. On the other hand, prints on large immovable objects that have been developed with a powder can best be preserved by “lifting.” The most popular type of lifter is a broad adhesive tape similar to clear adhesive tape. When the powdered surface is covered with the adhesive side of the tape and pulled up, the powder is transferred to the tape. Then the tape is placed on a properly labeled card that provides a good background contrast with the powder.

A variation of this procedure is the use of an adhesive-backed clear plastic sheet attached to a colored cardboard backing. Before it is applied to the print, a celluloid separator is peeled from the plastic sheet to expose the adhesive lifting surface. The tape is then pressed evenly and firmly over the powdered print and pulled up (see Figure 16–23). The sheet containing the adhering powder is now pressed against the cardboard backing to provide a permanent record of the fingerprint.

Digital Imaging for Fingerprint Enhancement When fingerprints are lifted from a crime scene, they are not usually in perfect condition, mak- ing the analysis that much more difficult. Computers have advanced technology in most fields, and fingerprint identification has not been left behind. With the help of digital imaging software, fingerprints can now be enhanced for the most accurate and comprehensive analysis.

Creating Digital Images Digital imaging is the process by which a picture is converted into a digital file. The image pro- duced from this digital file is composed of numerous square electronic dots called pixels. Images composed of only black and white elements are referred to as grayscale images. Each pixel is as- signed a number according to its intensity. The grayscale image is made from the set of numbers to which a pixel may be assigned, ranging from 0 (black) to 255 (white). Once an image is digi- tally stored, it is manipulated by computer software that changes the numerical value of each pixel, thus altering the image as directed by the user. Resolution reveals the degree of detail that can be seen in an image. It is defined in terms of dimensions, such as 800 � 600 pixels. The larger the numbers, the more closely the digital image resembles the real-world image.

The input of pictures into a digital imaging system is usually done through the use of scan- ners, digital cameras, and video cameras. After the picture is changed to its digital image, several methods can be employed to enhance the image. The overall brightness of an image, as well as the contrast between the image and the background, can be adjusted through contrast-enhancement methods. One approach used to enhance an image is spatial filtering. Several types of filters pro- duce various effects. A low-pass filter is used to eliminate harsh edges by reducing the intensity

FIGURE 16–23 “Lifting” a fingerprint. Courtesy Sirchie Fingerprint Laboratories, Youngsville, N.C., www.sirchie.com

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difference between pixels. A second filter, the high-pass filter, operates by modifying a pixel’s numerical value to exaggerate its intensity difference from that of its neighbor. The resulting effect increases the contrast of the edges, thus providing a high contrast between the elements and the background.

Analyzing Digital Images Frequency analysis, also referred to as frequency Fourier transform (FFT), is used to identify periodic or repetitive patterns such as lines or dots that interfere with the interpretation of the image. These patterns are diminished or eliminated to enhance the appearance of the image. Interestingly, the spacings between fingerprint ridges are themselves periodic. Therefore, the con- tribution of the fingerprint can be identified in FFT mode and then enhanced. Likewise, if ridges from overlapping prints are positioned in different directions, their corresponding frequency information is at different locations in FFT mode. The ridges of one latent print can then be enhanced while the ridges of the other are suppressed.

Color interferences also pose a problem when analyzing an image. For example, a latent fin- gerprint found on paper currency or a check may be difficult to analyze because of the distract- ing colored background. With the imaging software, the colored background can simply be removed to make the image stand out (see Figure 16–24). If the image itself is a particular color, such as a ninhydrin-developed print, the color can be isolated and enhanced to distinguish it from the background.

Digital imaging software also provides functions in which portions of the image can be examined individually. With a scaling and resizing tool, the user can select a part of an image and resize it for a closer look. This function operates much like a magnifying glass, helping the examiner view fine details of an image.

An important and useful tool, especially for fingerprint identification, is the compare func- tion. This specialized feature places two images side by side and allows the examiner to chart the common features on both images simultaneously (see Figure 16–25). The zoom function is used in conjunction with the compare tool. As the examiner zooms into a portion of one image, the software automatically zooms into the second image for comparison.

FIGURE 16–24 A fingerprint being enhanced in Adobe Photoshop. In this example, on the left is the original scan of an inked fingerprint on a check. On the right is the same image after using Adobe Photoshop’s Channel Mixer to eliminate the green security background. Courtesy Imaging Forensics, Fountain Valley, Calif., www.imagingforensics.com

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Although digital imaging is undoubtedly an effective tool for enhancing and analyzing images, it is only as useful as the images it has to work with. If the details do not exist on the original images, the enhancement procedures are not going to work. The benefits of digital enhancement methods are apparent when weak images are made more distinguishable.

FIGURE 16–25 Current imaging software allows fingerprint analysts to prepare a fingerprint comparison chart. The fingerprint examiner can compare prints side by side and display important features that are consistent between the fingerprints. The time needed to create a display of this sort digitally is about 30 to 60 minutes. Courtesy Imaging Forensics, Fountain Valley, Calif., www.imagingforensics.com

Fingerprints are a reproduction of friction skin ridges found on the palm side of the fingers and thumbs. The basic principles underlying the use of fingerprints in criminal investigations are that (1) a fingerprint is an individual characteristic because no two fingers have yet been found to possess identical ridge characteristics, (2) a fingerprint remains unchanged during an individual’s lifetime, and (3) fingerprints have general ridge patterns that permit them to be systematically classified. All fingerprints are divided into three classes on the basis of their general pattern: loops, whorls, and arches.

Fingerprint classification systems are based on knowledge of fingerprint pattern classes. The individuality of a fingerprint is not determined by its general shape or pattern, but by a careful study of its ridge characteristics. The expert must demonstrate

a point-by-point comparison in order to prove the identity of an individual. AFIS aids this process by converting the image of a fingerprint into digital minutiae that contain data showing ridges at their points of termination (ridge endings) and their branching into two ridges (bifurcations). A single fingerprint can be searched against the FBI AFIS digital database of 50 mil- lion fingerprint records in a matter of minutes.

Once the finger touches a surface, perspiration, along with oils that may have been picked up by touching the hairy portions of the body, is transferred onto that surface, thereby leaving an impression of the finger’s ridge pattern (a finger- print). Prints deposited in this manner are invisible to the eye and are commonly referred to as latent or invisible fingerprints.

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Virtual Forensics Lab

Fingerprinting To perform a virtual fingerprinting lab, go to www.pearsoncustom .com/us/vlm/

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Visible prints are made when fingers touch a surface after the ridges have been in contact with a colored material such as blood, paint, grease, or ink. Plastic prints are ridge impressions left on a soft material, such as putty, wax, soap, or dust. Latent prints deposited on hard and nonabsorbent surfaces (such as glass, mirror, tile, and painted wood) are preferably developed by the application of a powder; prints on porous surfaces (such as paper and cardboard) generally require treatment with a chemical. Examiners use various chemical methods to visual- ize latent prints, such as iodine fuming, ninhydrin, and Physi- cal Developer. Superglue fuming develops latent prints on nonporous surfaces, such as metals, electrical tape, leather, and plastic bags. Development occurs when fumes from the glue adhere to the print, usually producing a white latent print.

The high sensitivity of fluorescence serves as the under- lying principle of many of the new chemical techniques used to visualize latent fingerprints. Fingerprints are treated with chemicals that induce fluorescence when exposed to a high- intensity light or an alternate light source.

Once the latent print has been visualized, it must be per- manently preserved for future comparison and for possible use as court evidence. A photograph must be taken before any further attempts at preservation are made. If the object is small enough to be transported without destroying the print, it should be preserved in its entirety. Prints on large immovable objects that have been developed with a powder are best preserved by “lifting” with a broad adhesive tape.

review questions

1. The first systematic attempt at personal identification was devised and introduced by ___________.

2. A system of identification relying on precise body measurements is known as ___________.

3. The fingerprint classification system used in most English-speaking countries was devised by ___________.

4. True or False: The first systematic and official use of fingerprints for personal identification in the United States was adopted by the New York City Civil Service Commission. ___________

5. The individuality of a fingerprint (is, is not) determined by its pattern.

6. A point-by-point comparison of a fingerprint’s ___________ must be demonstrated in order to prove identity.

7. ___________ are a reproduction of friction skin ridges.

8. The form and pattern of skin ridges are determined by the (epidermis, dermal papillae).

9. A permanent scar forms in the skin only when an injury damages the ___________.

10. Fingerprints (can, cannot) be changed during a person’s lifetime.

11. The three general patterns into which fingerprints are di- vided are ___________, ___________, and ___________.

12. The most common fingerprint pattern is the ___________.

13. Approximately 5 percent of the population has the ___________ fingerprint pattern.

14. A loop pattern that opens toward the thumb is known as a(n) (radial, ulnar) loop.

15. The pattern area of the loop is enclosed by two diverg- ing ridges known as ___________.

16. The ridge point nearest the type-line divergence is known as the ___________.

17. All loops must have (one, two) delta(s).

18. The approximate center of a loop pattern is called the ___________.

19. If an imaginary line drawn between the two deltas of a whorl pattern touches any of the spiral ridges, the pat- tern is classified as a (plain whorl, central pocket loop).

20. The simplest of all fingerprint patterns is the ___________.

21. Arches (have, do not have) type lines, deltas, and cores.

22. The presence or absence of the ___________ pattern is used as a basis for determining the primary classifica- tion in the Henry system.

23. The largest category (25 percent) in the primary clas- sification system is (1/1, 1/2).

24. A fingerprint classification system (can, cannot) un- equivocally identify an individual.

25. True or False: Computerized fingerprint search systems match prints by comparing the position of bifurcations and ridge endings. ___________

26. A fingerprint left by a person with soiled or stained fin- gertips is called a(n) ___________.

27. ___________ fingerprints are impressions left on a soft material.

28. Fingerprint impressions that are not readily visible are called ___________.

29. Fingerprints on hard and nonabsorbent surfaces are best developed by the application of a(n) ___________.

30. Fingerprints on porous surfaces are best developed with ___________ treatment.

31. ___________ vapors chemically combine with fatty oils or residual water to visualize a fingerprint.

32. The chemical ___________ visualizes fingerprints by its reaction with amino acids.

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