Trace evidence
Trace evidence refers to small, often microscopic materials that can be found at crime scenes and are crucial in establishing connections between suspects and criminal activities. This type of evidence typically includes hair, fibers, paint residues, dust, and bodily fluids, all of which can provide significant insights during investigations. The foundation of trace evidence analysis is built on Locard's exchange principle, which states that whenever two objects come into contact, they transfer particles to one another, leaving behind trace evidence that can be analyzed.
Commonly analyzed trace materials include hair, which might indicate a struggle, and fibers from clothing or carpets that can link victims and assailants. Additionally, paint chips can help identify vehicles, while dust and dirt can reveal a person’s recent locations. Other critical forms of trace evidence are bodily fluids, bite marks, and tool marks, each of which can provide unique identifiers related to individuals involved in a crime. Emerging research suggests that an individual's microbiome may also be a promising avenue for identifying suspects, as distinctive bacteria can remain viable for several days after contact. Overall, while trace evidence alone may not be sufficient to convict individuals, it plays an essential role in piecing together events and linking suspects to crime scenes.
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Trace evidence
Definition: Forms of physical evidence at crime scenes that are usually not visible to the naked eye
Significance: Trace evidence is an integral part of many police investigations because it employs scientific methods to establish direct links between suspects and crime scenes. Trace evidence can provide powerful clues for reconstructing crimes for which little other evidence exits.
During the early part of the twentieth century, Edmond Locard, a French scientist who directed the world’s first crime laboratory, articulated a principle of criminology that became known as Locard’s exchange principle. It holds that when two physical objects come into contact with each other, each leaves particles on the other. Locard observed that no crime scene can ever be completely free of evidence; although no evidence may be seen by the naked eye, some evidence is always left behind. Locard’s principle has served as the foundation for the forensic study of trace evidence.
Trace evidence alone may be inadequate to build cases against criminal suspects, but it can establish links between suspects and crime scenes or victims of crimes. The types of trace evidence analyzed most often at crime scenes are hair and fibers. Large amounts of hair found at a crime scene often indicate a struggle; they are commonly found on floors near weapons or points of contact between victims and their assailants. Rooted hair may provide DNA evidence. Carpet and clothing fibers are also commonly found in places where assailants and their victims come into contact with one another.
Other important types of trace evidence include paint residue, dust and dirt, and firearm residues. Chips of paint from vehicles, doors, and furniture are often found on weapons and clothes. Paint chips from vehicles can often be used to identify the vehicles’ production years and makes and models. Dust and dirt residue can be used to determine places where people have been, where they reside, and what kinds of animals they have come into contact with. Other, similar organic materials that form trace evidence might include pollen, wood, or fungal spores. Residues left by ammunition can indicate whether suspects have fired firearms, but tests for such residues must be conducted within six hours of the time that weapons are fired. Explosive materials and propellants can likewise leave behind identifiable residues.
Another broad category of trace evidence is used to identify the characteristics of persons involved in crime scenes. This type of evidence includes bodily fluids, bite marks and other wounds, shoe prints, and tool marks. Bodily fluids, which can provide useful evidence whether they are fresh, coagulated, or dry, include blood, semen, saliva, and sweat. The dryness of blood specimens can be used to estimate how much time has passed since crimes have been committed. Bite marks—which might be found on either victims or suspects—can be particularly useful in identifying suspects, as every person’s teeth leave unique impressions. Likewise, every person has a walk that leaves shoe prints that are unique because of both the way in which people walk and the distinctive wear on their shoes. Wounds made by weapons can often be used to determine the size, shape, and length of the weapons. Even tools leave identifying marks that can be used to link suspects with crime scenes.
Research also suggests that the microbiome—an individual's unique makeup of bacteria, both internal and external—may also one day serve as a reliable, cost-effective means of identifying suspected criminals. Each person leaves some of their bacteria behind wherever they go, and these microbes remain viable for up to seventy-two hours afterward. Furthermore, bacteria samples are comparatively easier to analyze than inorganic residues such as fibers or paint.
Bibliography
Houck, M. Mute Witnesses: Trace Evidence Analysis. San Diego, Calif.: Academic Press, 2001.
Houck, M. Trace Evidence Analysis: More Clues in Forensic Microscopy and Mute Witnesses. San Diego, Calif.: Academic Press, 2003.
Lee, H., H. C. Lee, and T. Timedy. Blood Evidence: How DNA Is Revolutionizing the Way We Solve Crimes. Cambridge, Mass.: Perseus Publishing, 2004.
Oaklander, Mandy. "A Strange New Way to Solve Crimes." Time 31 Aug. 2015: 44–47. Academic Search Complete. Web. 31 May 2016.
Stoney, David A., and Paul L. Stoney. "Critical Review of Forensic Trace Evidence Analysis and the Need for a New Approach." Forensic Science International 251 (2015): 159–70. Web. 31 May 2016.