Crime Scene Investigations

I admit that collecting evidence can be a hassle, but evidence is a critical component of every criminal case we handle. Sometimes, evidence is easy to find. Take, for example, the baggie of blue pills and the bottle of liquid methadone that I saw sitting on the center console of a guy's car when I got out to talk with him. Of course, they weren't his, but you get the point...some evidence doesn't take much effort to find.

Sometimes, though, tracking down evidence can be difficult. This is especially true for small pieces of evidence and in difficult light conditions. These are ideal conditions for trying to use a thermal imager (TI) to assist you. Since the TI sees heat, any residual heat on the object you seek may be transmitted to the TI and shown on the display.

Static Scenes

One type of crime scene we investigate is the static scene. A static scene is one that is stable; the location of the crime is confined within an area and the expected area of evidence is confined as well. A classic example of this would be a shooting at a neighborhood store. We would expect to find all of the shell casings, bullets, and the like all in the immediate vicinity of the store. While the suspect may have fled, we don't expect that he has left a trail of evidence behind him.

In these scenes, a thermal imager can help identify evidence that is hidden from the human eye. Shell casings, for example, may still be hot from the shooting. As such, they should be readily identifiable with a TI, regardless of the amount of light in the area. Holes in a wall, blood spatters, dropped clothing...all of these will have heat signatures that can be picked up by the TI.

Dynamic Scenes

The opposite of a static scene is a dynamic scene. These are the scenes where an evidence trail may be actively strewn across some distance. If we take the store shooting, but then assume the suspect was wounded during the crime, as the suspect flees, we expect him to leave a trail of evidence behind.

He may be discarding clothing that identifies him as the criminal; he may be bleeding and leaving a trail of blood droplets. He could be throwing his weapons near bushes, or ditching unwanted proceeds from the crime, such as the bag that held what he stole. Again, all of these items will have a different heat signature and may be viewable with a TI.

The same thought process applies to a dynamic scene caused by a fleeing suspect. Not only can the TI help us locate the suspect more quickly, we can walk back along his path and look for anything he may have dropped or tossed as he ran. In these situations, since the evidence was probably in a pocket or elsewhere close to his body, the evidence should have absorbed a substantial amount of body heat. As such, it should radiate its heat signature clearly to the TI, making it very easy to locate.

Scene Evaluation

Like much of police work, we sometimes end up in a gray area, unsure if a crime has been committed or not. I recently used a TI to help evaluate a potential crime scene, prior to trampling through it.

A fellow officer and I were dispatched to a report of a trash bag along the railroad tracks. The complainant said that she saw bones sticking out of it. Our sergeant responded as well. When we got there, there was no doubt that something was rotting near the tracks; the odor was atrocious. The complainant pointed down the tracks, saying that she'd found a black plastic trash bag and didn't know what was in it.

Since I have a little experience with TIs, I know that most black plastic trash bags are transparent to infrared. That means you can "see" inside the bag with a TI. When we approached the bag, you could tell there was something inside was about the right size to be a small dog. Or an infant. Clearly, one of these meant a littering charge, the other a homicide.

I took a look at the bag with the TI. The decomposing body radiated its heat through the bag, and it had the clear shape of a four-legged animal. This lowered our concern about how to treat the scene. We opened the bag and found a paw, confirming that it was an animal, not a human.


Tracking down small pieces of evidence, especially in poor lighting, can be a great challenge. Similarly, finding tossed evidence after a foot pursuit can be nearly impossible. However, if the items have any retained heat, a thermal imager can help locate the items quickly and easily.

And sometimes finding that one extra piece of evidence is all it takes to make a case.