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Crime Laboratories

Historical Perspective, Crime Laboratories, Fbi Crime Laboratory, Engineering Research Facility, Sniper Attacks, Solving Old MysteriesModern forensic investigations

Crime laboratories offer forensic science services to the criminal justice system. Forensic science applies scientific testing methods and the latest technologies to collect, preserve, process, and analyze evidence. Proof of guilt or innocence is frequently determined by the results of forensic evidence.

Forensic science is a combination of many kinds of knowledge, some of which have existed, however primitive, for centuries. These include weapon identification, fingerprinting, document analysis, chemical identification, and trace analysis of hair and fibers. Two newer disciplines that have become major components of the twenty-first century crime laboratory are DNA analysis and explosive investigation.

The leading forensic laboratory in the world is at the Federal Bureau of Investigation (FBI), located 50 miles outside of Washington, D.C., in Quantico, Virginia. The FBI Laboratory moved from its site in downtown Washington, D.C., to its newly built facility in early 2003. The FBI Lab, with approximately 650 employees, partners with state and local crime laboratories throughout the country to solve criminal cases.

An FBI pathologist examines dried blood on a jacket. (© Anna Clopet/Corbis)

In this chapter a brief history of forensics is presented, followed by a description of the major types of forensic investigation used in crime labs at the local, state, federal, and private levels. Next is an in-depth look at the FBI Lab, followed by details of how crime lab investigations identified the individuals responsible for a series of sniper attacks that paralyzed the Washington, D.C., area in October 2002.

Firearms and toolmark identification

The identification of firearms and weapon-related evidence has played a critical role in crime investigation throughout the twentieth century. Firearms investigations involve the examination of fired bullets to determine what kind of weapon they were fired from. Examinations can eliminate various firearms until a match between bullet and weapon is made. All pistols, revolvers, and rifles contain "rifling" in the bore, the long protruding part through which a bullet travels. Rifling consists of grooves and markings on the inside surface of the bore. These markings have a right or left twist that varies with each manufacturer.

Rifling studies the path of a bullet according to each weapon's bore. A raised marking on the bullet will correspond to a groove in the bore. In addition to rifling characteristics, each fired bullet has tiny imperfections that reflect the imperfections in the bore. A test bullet fired from the gun investigators believe has been used in a crime must be identical to the bullet taken from the crime scene. If the test bullet and evidence bullet have the exact markings, then the crime weapon has been found. Together rifling and imperfections on the bullet can positively identify the weapon used to fire the bullet. These microscopic, or tiny, characteristics are as individualistic as human fingerprints.

Toolmark identification also falls to firearm specialists. Tools such as prying instruments, screwdrivers, and metal bars, or weapons like knives or axes leave identifying marks. Whether on a safe, a door, or a body, toolmarks help identify specific weapons. Tools generally have trace evidence, such as metal shavings, paint, or in the case of a human victim, blood or other biological substances. In assisting the investigation of a crime, both firearms and tool identification add specific information to a body of evidence.

A firearm and toolmark examiner shows an extracted bullet that was fired into a bullet recovery tank. (AP/Wide World Photos)

Latent prints

Latent prints refer to fingerprints, palm prints, and the footprints that are not visible to the unaided eye but can be recovered from a crime scene for study. Forensic print specialists retrieve the prints using powders, chemicals, or special lighting. Gathered latent prints are compared against those of suspects, or if no suspects have been identified, they are compared to hundreds of thousands of prints in a computer database.

Fingerprint identification and comparison has been widely used in criminal investigations since the 1920s and early 1930s. A young J. Edgar Hoover (1895–1972), who worked at the Bureau of Investigation, created a fingerprint database in 1924. At that time 810,188 fingerprint records from Leavenworth Penitentiary in Kansas and the National Bureau of Criminal Identification were combined to form the first FBI file.

Questioned Documents

Identifying who wrote a document and the time a document was written depend on analyzing writing characteristics, ink types, and typewriter, printer, and photocopy characteristics. The Questioned Documents department of the crime laboratories also restores documents that are damaged or have faded over time. One major function of document analysis is to determine forgeries, copies or imitation documents created with the intention of pretending they are authentic documents. Log books, letters, diaries, various legal contracts, checks, wills, notes left at a crime scene, and medical records are typical documents that are analyzed. Chemical analysis of ink, including its age, is used to determine changes, additions, or the rewriting of documents.


One of the oldest and largest sections of a forensic crime lab is the chemistry department. The chemistry department undertakes the demanding task of identifying most solids or liquids crime investigators ask them to analyze. Bullet lead, metals from a disaster scene, paint chips from automobiles and structures, dyes, and biological specimens to test for illegal drugs, prescription drugs, alcohol, poisons, and food products all are examples of substances analyzed.

Crime labs generally divide the chemistry department into various parts. Metallurgy units analyze metals for strength, corrosion, or evidence of being tampered with. Paints are analyzed in another unit. Most chemical analyses are preformed on a wide array of instruments that must be maintained and quality controlled to assure accurate results. Maintenance and quality control are carried out by yet another chemistry section. These chemistry sections provide key pieces to crime puzzles.

The toxicology unit, one of the largest chemistry sections, analyzes biological specimens. Toxicology is the study of toxic or poisonous substances, substances that can produce harm or death to any individual who takes them. The toxicity, or deadliness, depends on the amount ingested by the victim. For example, prescription drugs taken as directed do not generally cause a toxic effect, but overdoses of the same drug can lead to death. Toxins or poisons can be manmade or occur naturally in nature like arsenic.

One of the oldest and largest sections of a forensic crime lab is the chemistry department. The chemistry department undertakes the demanding task of identifying most any solid or liquid crime investigators ask them to analyze. (© Steve Chenn/Corbis)

For centuries, poisoning was a favored method of murder, and the study of poisons has gone on just as long. In the twentieth and twenty-first centuries, although poison is still used in a few murders and suicides, accidental poisonings are more common. The American Association of Poison Control Centers states that approximately two million poisoning accidents occur every year.

Trace evidence

Trace evidence refers to microscopic or larger materials, commonly hairs or fibers, which are transferred from person to person or object to object during a crime. Examples include human hair, animal hair, wood fibers, clothing fibers, carpet or car seat fibers, rope, and feathers. Also analyzed in a trace evidence laboratory are dental specimens. Forensic dentistry is known as odontology. Physical anthropologists, who study the physical makeup of humans through millions of years, analyze skeletal remains in the trace evidence lab.

A forensic scientist examines traces taken from a suspect vehicle in a hit-and-run fatality case. Trace evidence refers to microscopic or larger materials, commonly hairs or fibers, which are transferred from person to person or object to object during a crime. (AP/Wide World Photos)


Deoxyribonucleic acid, or DNA, is the substance that chromosomes are made of. Chromosomes are long connected double strands of DNA that have a structure resembling a twisted ladder. Along the chromosome strands are genes, or the genetic code unique to every person (except in special cases such as identical twins who have the same genetic code). Individuals inherit one strand of the DNA chromosome from their mother and one strand from their father. The paired chromosomes, one from the mother and one from the father, are located in the nucleus of each individual's cells.

After specific laboratory treatment, DNA double strands separate into single strands. When photographed with the aid of high power microscopes and computers, a single DNA strand appears as a long row of light and dark bands that look something like the bar codes on items for sale. The end result is that scientists can compare two DNA samples to see if the bands line up. If there is an exact match, the two samples came from the same person.

When samples from a crime investigation include blood, saliva, and other body fluids, chromosomal DNA is separated into single strands and the bar code of each strand becomes a DNA profile. If the evidence includes tiny or damaged quantities of DNA from hair, bones, teeth, and body fluids then A DNA lab worker performing the early stages of mitochondrial DNA extraction. (AP/Wide World Photos)
another kind of DNA testing can be analyzed and will also produce a unique banded DNA profile.


Explosive experts respond to the scene of an explosion or investigate the discovery of an undetonated (unexploded) bomb. They examine and identify parts of bombs. Even if an explosion occurred, the components used to build and detonate the bomb, although damaged, often can still be identified.

Chemicals, switches, wires, and detonators are all characteristic of the builder. The builder may be an individual or a terrorist group. The number of bomb response units has multiplied across the United States since the bombings in Oklahoma City in 1995 and the World Trade Center Towers in New York City in 1993. The FBI estimates that 85 percent of terrorist activities attempted toward U.S. citizens or interests at the beginning of the twenty-first century involved explosive devices.

Forensic analysis

The Latent Prints Unit has two categories of fingerprint holdings, criminal and civil. The criminal fingerprint file contains print records of about 47 million individuals who have been arrested and charged with a crime. The civil files have about 30.7 million print records. Civil prints are made of all employees of the federal government, all members of the U.S. military, those seeking employment in the banking and stock A forensic scientist studying latent fingerprints in front of a filtered green xenon light that is used to highlight chemical images captured on a digital camera. (AP/Wide World Photos)

market industry, individuals applying for U.S. citizenship, adopting a child, or working or volunteering at a child or senior daycare center.

The purpose of keeping the civil print database is to perform background checks searching for a criminal history. The FBI shows an average annual hit of 900,000 checks—or 900,000 prints each year submitted to the civil databases are identified persons with a criminal history record. Civil submissions also are checked against wanted persons files and a terrorist watch list.

The FBI receives approximately fifty thousand fingerprint submissions every day. It received a total of 17,736,541 submissions in 2003. Some 48 percent of these submissions were civil and 52 percent criminal. In July 1999 the FBI revolutionized its databases with the Integrated Automated Fingerprint Identification System (IAFIS). Previously, all prints arrived on paper fingerprint cards that had to be processed by hand. With the introduction of IAFIS, prints and pictures can be submitted electronically.

The IAFIS allows personnel of the Latent Print Unit to quickly process requests from law enforcement agencies all over the country for criminal background checks of people arrested or to check prints recovered at crime scenes against those on file. Many identifications have been made when law enforcement had no suspects. Other times suspicion can be lifted from innocent people and the real offender captured.

The Questioned Documents Unit also compiles a number of databases that can be crosschecked. These databases include a Fraudulent (bad) Check File, Bank Robbery Note File, Anonymous Letter File (threatening unsigned letters), and even a Shoeprint File. Three units provide laboratory analysis of evidence: Firearms-Toolmarks, Latent Prints, and Questioned Documents. The Firearms-Toolmarks Unit provides extensive firearm and toolmark identifications on weapons and tools used in crimes. They also restore serial numbers that have been altered or filed off of firearms to hinder identification.

Scientific analysis

The newest unit in the FBI Lab is the Chem-Bio Sciences Unit. Chem-Bio was added in 2003 for the forensic examination of hazardous materials. This unit works closely with the U.S. military, analyzing chemical and biological, even nuclear, substances and is developing portable identification systems to be taken to field locations where incidents have occurred.

The Chemistry Unit is one of the largest with secondary sections consisting of General Chemistry, Toxicology, Paints, Metallurgy, Elemental (analysis of elements such as lead, arsenic, and silicon in glass), and Instrumentation Operation and Support. The Paint Unit has samples of every color of paint used on U.S. automobiles dating back to the 1920s. Every auto manufacturer in the United States must send in paint samples every year. Most foreign manufacturers also submit samples. From tiny smudges of paint the FBI paint sleuths can identify the make, model, and year of any automobile.

DNA profile matches and checks identify murderers and sex offenders often when there had been no suspect. They also release innocent persons from suspicion. The three DNA divisions are the CODIS Unit, DNA Analysis Unit I, and DNA Analysis Unit II. CODIS stands for Combined DNA Index System. The several CODIS indexes provide (1) a database of DNA profiles from crime scene investigations, (2) a database of DNA profiles of persons convicted of felony sex and other crimes, and later added, (3) a missing persons DNA profile database.

Federal, state, and local forensic laboratories exchange DNA profiles of convicted offenders using CODIS's National DNA Index System (NDIS). The NDIS contained 1,566,552 DNA profiles of convicted offenders as of October 2003. DNA Analysis Unit I (nuclear DNA) and DNA Analysis Unit II (mitochondrial DNA) analyze samples from body fluids such as blood, saliva, semen, and from hair, bones, and teeth.

Forensic science support

The Counterterrorism and Forensic Science Research Unit carries out research activities for all units of the FBI Lab, introducing new and more precise scientific techniques. The constantly evolving methods of this unit help solve crimes and thwart terrorist actions.

The Special Photographic Unit provides a wide range of services such as crime scene photography, surveillance, photography from the air, and aerial mapping. The unit works with traditional photographic processes as well as with digital imaging. In addition, it provides maintenance for photographic equipment throughout the FBI Lab and supports personnel with ongoing training classes.

Operational response

Three units provide some of the FBI's most dramatic services: Bomb Data Center, the Evidence Response Unit, and the Hazardous Materials Response Unit. The Bomb Data Center provides the latest training, techniques, and equipment to local law enforcement bomb squads—those who are the first to respond to threats of explosive and biological weapons.

The Evidence Response Team Unit organizes the activities of Evidence Response Teams (ERTs), made up of FBI special agents trained in evidence recovery from incident sites. ERTs traveled halfway across the world to Piyadh, Saudi Arabia, after vehicle bombs destroyed residential buildings there on May 12, 2003. Twenty-three people, including nine Americans, died in those bombings. ERTs are available to assist foreign Members of the FBI Evidence Response Team prepare to make their way to a campsite believed to have been used by a bombing suspect. (AP/Wide World Photos)

countries when specially requested. ERTs carry out crime scene investigations including evidence collection, the preservation and documentation of that evidence, and also provide photography and fingerprinting.

The Hazardous Materials Response Unit coordinates specialized response teams trained to handle chemical, biological, radiological (radioactive), and nuclear substances. It also oversees national and international training to respond to such materials, and supports FBI response programs located throughout the country. The unit also deals with an increasing caseload of environmental crimes such as the illegal dumping of waste into the nation's waterways.

Operational support

Two units that provide operational support develop fascinating recreations of crime scenes by reconstructing not only buildings, but providing drawings of suspected individuals. The Investigative and Prosecutive Graphics Unit surveys crime scenes, then produces computerized animated scenarios of human movements and actions during a crime. They provide maps, floor plans, diagrams, and timelines, and they are the unit responsible for composite drawings of suspects from victim interviews. The unit can also reconstruct the appearance of individuals from skeletal remains.

The Structural Design Unit supports expert testimony in trials by providing three-dimensional models of crime sites, scale models of vehicles, and models of bomb devices. This unit also provides mannequins of victims for wound locations.

Examples of investigative aids from the Structural Design and Graphics units include the composite drawing of Timothy McVeigh, who bombed the Murrah Federal Building in Oklahoma City; a model of the Murrah Building before and after the bomb blast; a map of Columbine High School in Littleton, Colorado, site of a mass shooting in April 1999; and a model of the cabin where Ted Kaczynski, the notorious "Unabomber," lived in Montana. Until captured Kaczynski mailed sixteen bombs between 1978 and 1995 to selected individuals across the United States killing three and injuring twenty-nine.

For More Information


Federal Bureau of Investigation. FBI Laboratory 2003. Quantico, VA: U.S. Department of Justice, 2003.

Inman, David, and Norah Rudin. Introduction to Forensic DNA Analysis. Sarasota, FL: CRC Press, 1997.

Lane, Brian. Crime and Detection. New York: Alfred A. Knopf, 1998.

Smyth, Frank. Cause of Death: The Story of Forensic Science. New York: Van Nostrand Reinhold Company, 1980.

Web Sites

American Association of Poison Control Centers. http://www.aapcc.org (accessed on August 19, 2004).

Federal Bureau of Investigation (FBI). http://www.fbi.gov (accessed on August 19, 2004).

Human Genome Study Information. http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml (accessed on August 19, 2004).

Additional topics

Law Library - American Law and Legal InformationCrime and Criminal Law