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.
Tool Marks Not Distinguishable - While tools can leave tool marks when used as a tool they are all too often used as weapons as well. For example, a young college student was found brutally raped and murdered. Her skull crushed by repeated with a blunt object. The medical examiner concluded that the blows were most likely from a hammer but that there was so much damage to the skull it was impossible to develop a positive impression (in this case tool mark) of the weapon. Several suspects are developed but no one stands out initially. Two suspect are construction workers at a site on the campus where the young co-ed was found murdered. One suspect is an electrician the other a carpenter / sheetrock installer. Trace analysis of the victim's skull in the areas of the blunt force trauma reveal the presence of a gray-white chalk like material with particle of blue material in it. The material was collected and sent to the trace lab for chemical analysis.
Several Levels of Trace
An eyewitness comes forward placing the carpenter suspect in the vicinity of the victim the last time she is seen. Investigation of the carpenter leads investigators to name him as a prime suspect. His tool box at the job site reveals a large drywall hammer with similar gray-whitish material on it. This material also has the same blue flecks of material in it as those on the victims head. Lab analysis of this material concludes that it is calcium sulfate, or gypsum, as used in construction drywall material. While most drywall is white, the gray-blue color in this case comes from a special type drywall used in wet areas like bathrooms, it is called blue-board because of a special blue colored covering paper that is water resistant and used on the outer surface of the boards. This is the surface that is nailed up with a hammer or drill to attach the boards to stubs. The suspect had been installing similar material at the project for two weeks.
While this was a good circumstantial link of the suspect to the dead girl there was no direct proof that he or his hammer had actually committed the crime. Careful examination of the hammer when brought in as evidence initially revealed what appeared to be trace amounts of biological material in small striations along one side of the hammer. Thus material was preserved and upon microscopic analysis proved to be human brain tissue. DNA analysis of the trace material showed that it was identical to that of the young victim's DNA. The case was sealed at that point. A tool that had no left clear tool marks was now definitely linked to a crime and a suspect.
However, that brain traces material; could easily have been lost in the early days of the investigation if it had not been handled carefully. The material could have easily been dislodged from the hammer groves, or dried up and fallen out. It could have been missed altogether if not for a diligent analysis by a trained forensic trace technician.
