At a crime scene, there are often tiny fragments of physical evidence such as hairs, fibers from clothing or carpeting, or pieces of glass that can help tell the story of what happened. These are referred to as trace evidence, and can be transferred when two objects touch or when small particles are disbursed by an action or movement. For example, paint can be transferred from one car to another in a collision or a hair can be left on a sweater in a physical assault. This evidence can be used to reconstruct an event or indicate that a person or thing was present.
Careful collection of materials from a crime scene can yield a wealth of information about where a sample came from and how it helps to tell the story. Scientists examine the physical, optical and chemical properties of trace evidence and use a variety of tools to find and compare samples, and look for the sources or common origins of each item. Most test methods require magnification and/or chemical analysis.
The importance of trace evidence in the context of crime scene investigation is sometimes understated, taking a back seat to more individualized evidence such as DNA or fingerprints. Much can be learned about what happened at a scene through trace evidence, such as whether an item or body was moved or whether someone was assaulted from behind or the side. Trace evidence can include a wide variety of materials, but the most commonly tested are hair, fibers, paint and glass. Other, less frequently included items are soil, cosmetics and fire debris. Some laboratories will consider fire accelerants as trace and others will include them in chemistry, even though the same tests are conducted in both laboratories. For the purposes of this series, paint, glass, fiber, and hair will be included in the discussion.