Traces of Trace

Every encounter between two object transfers some material, no matter how small, between the two objects. Every student of law enforcement knows this of course as Locard's exchange principle. However, in many cases this exchange is not readily apparent. While hair, bodily fluids, fibers and bloodstains are routinely thought of as the items that are found on analysis for trace evidence they are not the only thing that can be left behind at a crime scene. Paint or coating materials, glass, wood or plastic particles, metal flakes, as well as, polymers and resins can constitute trace evidence.

Tool Mark and Trace Evidence

One area where these types of trace evidence can be found is when a tool or an object is used to commit the crime. This might represent a crow bar used to pry open a lock or break in to a safe, or an object used as a weapon to kill someone by blunt force trauma. An area where trace evidence can be an invaluable adjunct piece of evidence is where there is primary evidence of tool marks are left at a scene. When an object is used at a tool whether to bend, break or otherwise manipulate another object the tool leaves behind marks that are characteristic of its surface. This mark may be visible to the naked eye but more often than not they are invisible marks visible only under a microscope. Historically the microscope has been the stereo microscope or other low power optical microscope. However, this has changed in the recent years with the advent of the scanning electron microscope or SEM. This scope with resolution levels up to 200,000 times that of the optical microscope has allowed a greater level of detail to be developed when comparing tool marks to suspected tools (weapons).

While the tool mark in itself may be a powerful piece of evidence as presented to a jury it can be strengthened if additional trace evidence is also presented which is associated with the tool marks. This is especially true in cases where the tool mark evidence is not clear cut. As an example take a case where a crowbar is used to gain entry into a store that is robbed. The crowbar is found in a tool box in the back of a suspect's pickup truck. The entry was gained by prying the padlock off the delivery entrance door. Tool marks on the bent metal latch of the door match microscopically the striations on the crowbar. Paint from the building which is painted yellow is also found as trace evidence on the crowbar. This is good supporting evidence. However, let's take the story one step further.

Color Me Trace

Careful analysis by the forensic lab technician of the paint on the crowbar also indicates that there are particles of a green paint mixed in with it. Reanalysis of the building lock clasp and the loading dock door indicate that identical green paint particles exist there also. The building was painted with the same green paint prior to having been repainted yellow. When presented right by the prosecution this additional evidence adds greatly to the strength of the evidence at trial.