Trace evidence is left at a crime scene when one object touches another. This type of evidence is quite small, but large enough to measure. Some examples of trace evidence are, scrape marks, fingerprints, hair, fibers, soil, tool marks, paint chips, glass fragments, etc.
Investigators must use caution when entering crime scenes to avoid disturbing or destroying trace evidence. They must also take steps to avoid contaminating the crime scene by accidentally depositing things that could be mistaken for evidence, such as cigarette butts, hairs, and chewing gum.
Professor Edmond Locard certainly hit the nail on the head when he said,
“Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but 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.”
Since trace evidence can be so tiny, investigators often have to rely all special detection devices, such as alternate light sources (ALS) and magnification instruments, to help locate the items.
Once the evidence has been found investigators must then collect it in a manner that both preserves the integrity of the item and prevents contamination. They must also record the methods they used for later testimony.
Methods of trace evidence collection
Scraping – Sometimes, investigators use flat objects such as a spatula to scrape items from flat surfaces. The evidence is then transferred from the spatula to an evidence bag or other approved container.
Picking – Tools, such as forceps or needle-nose pliers are used to collect evidence. A good example is the removal of a single hair from a shirt collar.
Vacuuming – Special vacuum cleaners (usually hand-held) are quite effective for collecting several items at once. These vacuums are equipped with filter traps that catch the trace evidence.
Lifting – This process is similar to lifting fingerprints. Investigators use the sticky side of special tape to collect evidence.
Combing – This technique is used to comb the hair of an individual to collect any debris or other foreign objects, such as hair or dandruff deposited by a killer, or rapist.
Clipping – Clipping normally refers to the trimming of a victim’s fingernails to retrieve tissue and DNA of an attacker. (If the victim struggled with his attacker, he may have scratched the killer).
Collected items should be packaged in clean, dry, paper containers, such as envelopes or paper bags. Petri dishes may also be used in some cases.
Trace evidence collection kit