Defining Trace Evidence

Pet hairs and fibers in a vehicle

“Trace Evidence” is a term that is commonly used by detectives and forensic scientists but may not be widely understood by others. The term gained prevalence in a series of three papers written by Edmond Locard in 1930 and published in the American Journal of Police Science. The papers define what has become known as the Locard Theory of Exchange. Basically, the theory holds that whenever two things come into contact with one another they will likely leave “traces” of their presence with each other. Imagine a man walking across a muddy field. He leaves behind shoe print impressions and in return the muddy soil will cling to his shoes. That scenario creates an “exchange” between the scene and the suspect. obviously, the more activity and participants in a given crime scene, the greater the potential for such exchange.

I’m not wedded to any particular definition but I think many criminalists would agree that when they think of “trace evidence” they imagine small items normally examined with a microscope. Others might also include impression evidence from shoes and tires but, as an author, you needn’t concern yourself with parsing it all out. I have previously described how criminalists “touch” evidence to minimize any exchange and/or destruction of this valuable evidence. But, detectives rely on this exchange to prove connections between the suspect, victim, and crime scene. As authors you can use various types of trace evidence to drive an investigation and challenge your characters. Trace evidence can be divided into several categories. Here are a few (of many) examples:

Biological:

• DNA (blood, semen, saliva)
• Hairs (human, pet, livestock)
• Fingerprints
• Stomach contents (vomit)

Physical:

• Synthetic fibers (clothing, carpet)
• Paint chips
• Safe insulation
• Glass fragments

Natural:

• Botanical (pollen, seeds, plant fragments)
• Geological (soil, sand)

I will soon post some articles describing how and where CSIs search for this trace evidence so keep checking back!