Trace evidence

Trace evidence is created when objects make contact, and material is transferred. This type of evidence is usually not visible to the eye and requires specific tools and techniques to be obtained. Due to this, trace evidence is often overlooked, and investigators must be trained to detect it. This type of evidence can link a victim to suspects and a victim or suspect to the crime scene. The importance of trace evidence in criminal investigations was shown by Edmond Locard in the early 20th century, with his exchange principle, that every contact leaves a trace. Since then, forensic scientists use trace evidence to reconstruct crimes and to describe the people, places, and things involved in them. Studies of homicides published in the forensic science literature show how trace evidence is used to solve crimes. There are three general categories in which forensic science uses trace evidence. It can be used for investigative aids, associative evidence, and in-scene reconstructions. In terms of investigative aids, trace evidence can provide information to determine the origin of a sample and determine the manufacture date of the material, all of which can limit potential suspects in a case. Associative evidence can associate with or link victims or suspects to a crime scene. For reconstructions, trace evidence can provide information to understand how a crime occurred or the events that occurred before the crime. Vehicular accident reconstruction relies on some marks to estimate vehicle speed before and during an accident, as well as braking and impact forces. Fabric prints of clothing worn by pedestrians in the paint and/or road grime of the striking vehicle can match a specific vehicle involved in a hit-and-run collision. Such traces are also known as "witness marks", especially in engineering and may be critical in understanding how a product failed. A typical witness mark could be an impact depression which broke a product, especially if that mark can be matched to the product which made the impact such as a hammer or nail.