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Trace evidence usually consists of particles, mostly microscopic, left on a crime scene by a perpetrator. The examination of trace evidence by forensic specialists often assists in identifying the source of a crime, a suspect, or a victim. Most people unknowingly leave trace evidence on the scene of a crime, and it is often difficult to remove. There are various forms of minute evidence that forensic teams use to corroborate additional findings. It is rare that trace evidence is used alone to support a case in a court of law.
Hairs and fibers can be key factors in identifying a suspect or victim, and are often transferred between the two. There are a multitude of elements of hair that can help forensic engineers during the identification process. Fibers are important because they can be transferred from vehicles, clothing, carpeting, and other fabric-based sources to the crime scene, or can be extracted from the scene and be found on a suspect. This portion of trace evidence can narrow down a search based on already known facts.
Forensic teams use a complex system to determine needed information from trace evidence to narrow down the unknowns and support other discoveries. Paint and dirt are also vital discoveries on a crime scene. Forensic science can determine the make and model of a car used to commit a crime or the type of paint from a crime scene could be discovered on a suspect's possessions. Dirt also leaves an imprint on a crime scene, and can be analyzed to determine where a suspect may live, work, or where he or she was prior to the crime.
The most essential category of trace evidence is biological, especially fluids and blood. Fluids can include sweat, semen, and saliva, which are often analyzed to identify deoxyribonucleic acid (DNA) and matched with a suspect's genetic code. Aside from its DNA importance, blood is highly valuable in recreating a crime scene and determining the time the act took place. Forensic photography is used to capture blood splatter patterns, which allows detectives to visualize the specifics of a crime.
Edmond Locard, who studied and experimented with forensic law in the early 1900s, proved that there was an exchange of evidence during a crime. This theory became Locard's Exchange Principle, which is the basis for trace evidence and other forensic discoveries. The microscopic details found during a criminal investigation usually help tie the larger evidence together and give additional insight into the behavior and identity of a suspect.