With the advent of the use of forensic DNA analysis in recent years, trace evidence has increasingly become an overlooked aspect of forensic science. In casework however, quite often little or no probative biological evidence can be found. It is in these instances that trace evidence still remains a critical, if not sole, link in solving the case.
Historically, trace evidence has been the discipline of forensic science that deals with the minute transfer of materials based on the theory that when two objects come in contact, each one would leave with something from the other. This is also sometimes referred to as Locard’s exchange principle.
Crime scene reconstruction is based heavily on the firm belief in Locard’s exchange principle. In shooting reconstruction the principle is extended to the belief that when two objects come in contact with each other, they can deform or damage one another. As an example, a bullet that deflects off of the concrete would have rough damage to its surface vs hitting a smooth metal surface where the damage would result in smooth striations. Also, the bullet would carry some pieces of concrete with it after the contact. (source: https://www.sciencedirect.com/topics/social-sciences/exchange-principle)
“Whether he wants it or not, wherever he touches, whatever he leaves, even without consciousness, will serve as a silent witness against him. His fingerprints or his footprints, his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value.” – Paul L. Kirk, Crime Investigation; physical evidence and the police laboratory
The role of trace evidence in forensic science is to provide links between:
- The victim and suspect
- The victim and scene
- The suspect and scene
In the absence of biological evidence, trace evidence may be the only evidence available. The handling and analysis of trace evidence requires care and specialized techniques. The types of equipment used to analyze trace evidence include stereomicroscope, scanning electron microscopy, polarized light microscope, comparison microscope, Fourier transform infrared spectrophotometer (FTIR), and gas chromatograph/mass spectrometer (GC/MS). Various dyes are also used to process gun-shot residue (GSR) for distance determination to provide estimated distance between the shooter and the victim.
Trace evidence analysis aims to identify, compare and individualize the source of evidence to aid in crime scene reconstructions. It can provide proof of a criminal act, provide new avenues of investigation and either refute or support a timeline or theory.
Items of evidence that qualify for trace analysis are:
- Hairs: determination of species, damage, disease, poisoning, DNA, etc.
- Fibers: natural, synthetic, textile or carpet
- Gunshot residue: distance determination based upon GSR
- Glass: composition, direction of force, sequencing
- Fire debris: ashes examined to retrieve the cause of fire (natural or chemically induced)
- Low explosives: presence of explosives and type determined
- Paints and polymers: determination of vehicle make/model in hit and run cases
- Footwear, tire and fabric impressions: crime scene reconstruction
- Toolmarks: striations caused by certain tools examined using comparison scope
- Physical fits: broken pieces aligning perfectly for reconstruction
- Ink: composition
- Automobile lamps: whether the lamps were on during an accident
- Inorganic and organic composition: wide array of substances including poisons
Trace evidence recovered from crime scenes can provide powerful leads for law enforcement. It can assist not only in identifying a suspect but in revealing strong associations between the suspect and the crime. In Ohio, a piece of plastic the size of a quarter helped police solve a 2000 murder. In New Jersey, DNA recovered from a small piece of electrical tape helped law enforcement agents find two people who smuggled contraband into a correctional facility with drones. These examples show that trace evidence remains relevant tools for law enforcement and prosecutors, even as biological and digital evidence gain prevalence.