Trace Evidence Collection
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
Fantastic post, Lee! I love the reminder about placing items in clean, dry paper envelopes and bags. I tend to cringe when I see things placed into plastic bags on the tube.
Another great post, as usual!
As always, great post. I’ve been busy — writing (finally), and quitting my day job (finally). I love these informative posts, especially with the pictures and terminology I can use in my writing. I don’t think my readers appreciate the technical terms like “do-hickie” and “thingie” but posts like this can add realism to the work. Again, thanks for keeping this blog.
And, as you were talking of drunk drivers not so long ago, that was the topic of last night’s Civilian Police Academy class. I’m sure most readers here are well aware of the dangers to everyone when impaired drivers are on the road, but the statistics were … (sorry) sobering. I posted them on my blog, should anyone be interested.
I used to watch the TV series “The Practice” but became disenchanted with it after awhile. One of the things that bugged me is that there seemed to be more cases than there should have been where they claimed that there was no trace evidence of the criminal that could be found at the crime scene.
That makes it easier in a drama to claim that there was no evidence to implicate nor exclude the defendant and then it all depended on alibis, testimony, etc.
It began to bug me as the easy way out for the writers of the series.
As well as the sheer number of women who were tortured and murdered by serial killers in Boston. I got tired of plotlines that were dependent upon what I consider to be gender-based hate crimes and did not find it to be entertaining.
My favorite episodes were ones that took topical disputes in law and demonstrated how both sides argued their cases. One I remember distinctly was in trying to prevent the execution of someone convicted of rape whose evidence from trial had been misplaced. Once it was found the lawyers argued to have DNA probes run on it since that technology had not been available at the time of the trial.
At first it was not allowed and the execution was still going to take place. Finally, after appeal the testing was performed and it exonerated the man and he wound up being released. That also meant that the real killer was never found.
The thing I liked the best about that episode was discussing why prosecutors would be opposed to post-conviction testing of DNA if it might overturn wrongful convictions.