Forensic science is the use of scientific principles and methods to analyze material connected with a crime. Such material is called physical evidence. It may include documents, drugs, fibers, fingerprints, hair, and soil. For example, a forensic scientist may connect glass splinters or a gun found on a suspect to broken glass or a bullet taken from the scene of a crime.
For much of the history of law, criminal trials depended upon evidence given by people who witnessed the crime. Rarely was an attempt made to find physical evidence at the crime scene or from the criminal. Beginning in the 1800’s, a small number of scientists began using physical evidence in criminal cases.
In the early 1900’s, the French physician Edmond Locard developed the principle that would become central to forensic science. In what is known today as the Locard exchange principle, he stated that every contact, no matter how slight, causes a transfer of material. Thus, a criminal always leaves some amount of material at a crime scene. In addition, some amount of material from a crime scene will remain, for a time at least, on a criminal. Forensic scientists examine this transferred material.
Forensic science is largely based on comparisons. Characteristics of items are compared to link them to a common source. The strength, or value, of the evidence depends on the certainty of the match. For example, forensic scientists may compare a piece of paint from the scene of a hit-and-run accident with the paint from a suspect’s vehicle. They try to determine how many characteristics of the paint chip are the same as the paint from the vehicle. They examine and compare the size and shape of the chip, its color and chemical composition, and the number of layers of paint. The more characteristics that match, the stronger the evidence is.
Forensic scientists most often work at crime laboratories. There are thousands of crime labs around the world. They range from those run by a government—such as the large, well-equipped lab of the United States Federal Bureau of Investigation (FBI) in Quantico, Virginia—to smaller local laboratories. Some labs specialize. The U.S. Internal Revenue Service crime lab in Chicago concentrates on the examination of documents. The U.S. Fish and Wildlife Service Forensics Laboratory in Ashland, Oregon, works only on crimes against endangered wildlife.
Crime scene investigations
After a crime, police officers secure (seal off) the crime scene as quickly as possible. They establish the boundaries of the scene and mark them with special tape or barriers. After enclosing the area, the officers permit only authorized people access. Investigators then systematically search the scene for evidence. They collect and package any object that might be related to the crime and transport it to the crime lab. They take precautions to avoid disturbing or contaminating the evidence, which would harm the investigation.
Generally, trained law enforcement officers or crime scene investigators gather evidence at a crime scene. Forensic scientists seldom go to the crime scene unless an unusual type of evidence is involved that requires special handling.
General kinds of evidence
Evidence can be classified into several general categories. They include direct and indirect evidence and class and individual evidence.
Direct and indirect evidence.
Direct evidence, also called testimonial evidence, is a verbal description of a crime by a witness. For example, a witness may observe a stabbing and describe the incident.
Indirect evidence may lead someone to conclude that an event occurred or that a fact is true, but the evidence does not conclusively prove the event or fact. Indirect evidence linking an accused person to a crime might include fingerprints on a weapon or a victim’s blood on the clothing of the accused.
Indirect evidence is also called circumstantial evidence. It describes a circumstance, or event. The more circumstantial evidence confirms the event, the stronger the case against the accused.
Class and individual evidence.
There are two main types of physical evidence, class evidence and individual evidence. Most physical evidence is class evidence—material common to a group of similar objects that cannot be linked to one particular source. A paint chip from a hit-and-run accident would likely be class evidence. However, a chip that exactly fit a hole in the paint on the suspect’s vehicle could be individual evidence.
Individual evidence is physical evidence that can be related to only one source. For example, most scientists consider DNA (deoxyribonucleic acid) to be individual evidence. DNA is genetic material found in all living cells. Experts can match some samples of DNA to a particular person with a mathematical certainty of billions to one. It is extremely unlikely that any two unrelated individuals possess identical DNA. Investigators may compare DNA found at the crime scene with DNA from a suspected criminal. On the basis of this comparison, the suspect can be included or excluded as a possible source of the DNA found at the scene of the crime.
What a forensic scientist analyzes
Forensic scientists analyze various kinds of physical evidence. They may use biology, chemistry, computer science, earth sciences, mathematics, and physics in their work.
Fingerprints.
Humans have designs on the skin of the fingertips formed by small ridges. These designs are called friction ridges. The impressions made by dermatoglyphs are fingerprints. Fingerprints were used for identification in China about 3,000 years ago, but their scientific study began in the mid-1800’s. Scientists believe that no two people have the same fingerprints. Thus, fingerprints are classified as individual evidence.
Many of the fingerprints at a crime scene are latent (not visible to the eye). Investigators use special types of powders or chemicals to discover and develop latent prints. Then they digitize the prints—that is, translate the prints from their original form into a form that a computer can read. Digitizing prints enables scientists to enter them into a database that other scientists can search. The FBI’s Integrated Automated Fingerprint Identification System database, for example, contains millions of fingerprint records. It can compare prints so rapidly that a preliminary match can be made in minutes.
Impressions
occur when an object is pressed against or into a surface. Tires and footwear, for example, may leave impressions that can be developed and compared to a known source. Law enforcement experts have created large databases of tire and footwear impressions from nearly all manufacturers. Most impressions are considered class evidence. For example, a number of cars may have the same type of tire. The more worn and damaged the tire is, however, the more it will show unique features that can change it from class to individual evidence.
Firearms.
By examining a bullet and cartridge, forensic scientists can often identify the type and manufacturer of the weapon. Identification of the type of gun that fired a cartridge provides class evidence. Marks inside the gun barrel can, however, cause markings on a bullet as it is fired. Comparing a bullet from a crime scene to one fired from a known weapon can sometimes provide a match that is considered unique, and therefore individual evidence.
Controlled substances
are drugs or other chemicals whose possession and use is restricted by law. Law enforcement officials at the scene of an arrest can administer initial tests for such drugs as cocaine and marijuana. If the test is positive, officials deliver the material to the crime lab, where a forensic scientist confirms the initial identification.
Poisons
have been a common method of murder for centuries. In a crime lab, experts in poisons, called toxicologists, analyze substances to try to identify poisons. This analysis can be complex, requiring a knowledge of chemistry and medicine.
Blood
is often a by-product of violent crime. Sophisticated tests are needed to determine if a stain is blood and, if so, whether it is human or animal blood. The pattern of bloodstains may also aid in the reconstruction of a crime scene and provide key evidence.
Trace evidence
may include tiny amounts of fibers, glass and metal particles, hair, paint, plant material, or plastics. Sophisticated and expensive instruments may be required to identify trace evidence.
Uses for forensic science
Technological advances in forensic science have created more applications for the work of experts in this field. Forensic scientists have been called on, for example, to analyze the scene of an ancient crime. In 1991, when a 5,300-year-old mummy called the Ice Man was discovered in a glacier in the Italian Alps, forensic scientists analyzed both his clothing and corpse. DNA analysis led scientists to believe that the Ice Man had died after a fight, because he had the blood of four other individuals on his clothing and weapons. Scientists found a wound from an arrow in his back, but determined that the Ice Man’s cause of death was likely a blow to the head, which killed him before he could bleed to death from his arrow wound.
Forensic scientists may also be consulted following airplane crashes, natural disasters, terrorist acts, and crimes committed during wars. In addition to performing autopsies on and identifying the bodies, forensic experts analyze wreckage to try to determine what caused an accident or disaster. In 2009, scientists were able to use a few feathers found in a turbine to identify the species of bird that had entered a jet engine and forced a plane to land in the Hudson River.
Forensic science is used to analyze places in which war crimes took place. In July 1995, more than 7,000 Bosnian Muslim men and boys were massacred by Bosnian Serb forces in Srebrenica, in present-day Bosnia-Herzegovina. Later in 1995, as mass graves began to be discovered in Srebrenica, few of the thousands of bodies and body parts removed from the graves could be identified by traditional means. By using DNA analysis, experts were able to identify more than 6,000 of the dead and have been able to reunite skeletal remains that were buried in separate mass graves.
Careers in forensic science
Beginning forensic scientists should have at least a bachelor’s degree in biology, chemistry, physics, or a related natural science. A graduate degree in forensic science is also helpful.
Most forensic scientists are civilians who do not carry a weapon or have the power to arrest people. They do not usually search for clues at a crime scene. Instead, they work in laboratories, carefully and methodically examining material. A forensic scientist must often appear as an expert witness in the courtroom to justify his or her results.
