Searching for Trace Evidence Part 2: The Crime Scene

Bloodstained rock with broken glass

Trace evidence can provide powerful associations between the suspect, victim, and crime scene.   In a previous article on this topic, here,  I talked about the principle of exchange and why forensic scientits search for this type of evidence. This posting is going to briefly cover the types of trace evidence your characters might be searching for and collecting from a crime scene. Basically there are two broad categories of trace evidence a CSI looks for at a crime scene; “known” and “question”.  A known sample (also called a “standard”) is one that comes from a known (verifiable) source at the time of collection. Some examples include

• Carpet fibers
• Broken glass from a window at the point of entry
• Soil samples from the yard or garden
• Pet hair
• Fibers from a couch
• Pollen from flowers

These are samples that will be used to compare to trace evidence found on the suspect or areas occupied by the suspect.

Other samples are called “question” or “unknown”. These are samples of an unknown origin. These may have been deposited by the suspect or any other person or animal in the scene. This evidence is typically compared to known standards from the suspect, their residence, vehicle, or other places they are associated. These samples may also be compared to samples from secondary crime scenes.  Obviously, unknown samples in “private” crime scenes like the victim’s home may carry more legal weight than samples collected from public places like a restaurant or bank (unless collected from a non-public area like the vault).

• Suspect hairs
• Fibers from suspect vehicle, clothing, or apartment
• Soil samples dislodged from shoes or boots
• Skin
• Handwriting samples (like from a robbery or ransom note)

So now that you know the types of evidence we need to talk about how to collect them. Known samples like carpeting are typically collected by cuttings. I would usually cut a 6″ or so sized sample. The type of crime also must be considered. A CSI is not going to cut out carpet samples in a burglary because you’d be damaging property without even knowing if you had a suspect or samples on the suspect to compare. In a murder scene where the crime is much more serious and the carpeting may already be damaged and bloodstained then it’s a much easier call. Soil, hairs, fibers, and evidence containing fingerprints (like broken glass) is best collected in a paper bag or envelope so it can “breathe”.  You can also collect hairs and fibers with an evidence vacuum, lint roller, fingerprint tape, or forceps (tweezers).

Lint roller with hairs and fibers

So where do you look for suspect trace evidence at the crime scene? The exchange of trace evidence is much more likely at points of significant contact. One of the best places is the point of forced entry. If I break a window to get into a home there’s a good chance I may cut myself. I may also snag my clothing or head hair on the jagged pieces as I reach or crawl through the opening. The same is true of any other area I have to damage to gain access like an interior door or safe. Another great spot is an obstacle along the path of approach or egress. It’s very common to find hair and clothing fibers along the tops of wooden fences the suspect climbed over.  Investigators may also find head hairs on a stolen vehicle’s head rest or headliner.

Hairs on fence top

As you are describing your crime scene consider the principle of exchange and the types of trace evidence your detectives might be searching for. The more exotic your evidence (like a rare potted flower species) the more interesting the association may become. Or you may choose to pick something so common (like white cotton fibers) that it really doesn’t connect your suspect at all. You could even consider inserting trace evidence from some other “innocent” source that leads your detectives on a wild goose chase allowing your bad guy to escape detection for a while.