Using trace evidence to solve the crime in your story is a great way to delay the payoff and finger the guilty party. I’ve given a list of the most common types of trace evidence and a brief description of each, though they could all be a post on their own.
Clothing, leather, car interiors, carpet, and rope can all provide fibers for the investigators to discover and trace. The Mass Spectrometer is one tool used to identify the chemical makeup of a fiber. For example, carpet manufacturers’ and some fabric manufacturers’ product chemical signatures are in a database or researchable. This could narrow down the year, make, and model in the case of a car, or distribution information in the case of carpet and other fibers. The most common use of fibers, however, is to match them from the crime scene to the suspect or the suspect’s environment.
Plants have DNA just like we do, so seeds or pods can often be linked to a specific plant. This might tell investigators where the suspect or the victim spent time as plants are often geographically specific. Another use would be, for example, the discovery of seed pods in the suspect’s vehicle and matching them to the seed pods under the victim.
Larvae and living insects aren’t considered trace evidence, but webs, egg casings, insect parts, and evidence of bites or stings can yield evidence as to location, length of time the victim was at the dump site or crime scene, and even season if the crime occurred well before discover.
Humans and animals shed hair all the time, making hair one of the most common types of trace evidence. Once a suspect is identified, strands of hair can be matched. If the hair is pulled from the root, it may contain enough biological material to get a DNA sample. Pet hair found on or near the victim and also in the suspect’s environment (matched to a specific animal) can also boost the prosecutor’s case.
Fluids include bodily secretions, of course, but other fluids include tree sap, cleaning products, chemicals, beverages, oil and gas, and water (pond water, ocean water, etc.). While these give information about the crime, I’ve always thought it would be interesting to use a fluid as the breakthrough piece of evidence. Something totally out of place would stump investigators and extend the tension of the story.
I like this sort of evidence, and I’m intrigued about how to use particulates to good effect in a murder mystery. Here are four:
- Paint: In addition to binding chemicals, paint often includes a mineral or metal as a color ingredient. These can be matched to known paint products, vehicles, and other elements of locations. In the world of crime shows, I’ve seen it most often used to identify a vehicle, but wouldn’t it be fun to connect it to an artist’s studio or the statue in the town square?
- Soil: The great thing about soil is that it’s fairly localized depending on the particulates. Sandy, clay-based, loam, fertilized, mineral-rich . . . all give the investigators information they might not otherwise discover.
- Metals: Paint, vehicles, tools, clothing fasteners, soil, weapons, and much more, contain metals that can be traced and/or matched to the crime scene, dump site, or suspect’s environment. We see it used most often to identify, again, a vehicle, but flecks of gold might indicate a manufacturing process, and copper shavings might point to the plumbing or construction industry.
Trace evidence offers so many opportunities for the writer and suspense for the reader. It’s by far my favorite type of evidence to include in stories.
The last post in the series is DNA and Biologics. See you then!
How many ways can you think of to stump the reader and detective with trace evidence?
Other posts in this series:
Crime Scenes for Writers: Trace