Our primary job as crime scene investigators is always the same: to tie a suspect to the scene. The way to accomplish this goal is to collect as much evidence as possible from the scene and the suspect.

The key point to keep in mind is that the scene will dictate what you need from the suspect. For example, if you find a pair of latex gloves at the scene, process the inside of the gloves for prints, then you will need to swab the hands of the suspect for traces of the powder from those gloves. If the victim was shot, test the suspect for gunshot residue. Keep in mind that this evidence is very fragile, so you can only test for residue if you apprehend the suspect within four hours of the crime. If you find blood at the scene, collect all of it, since some may belong to the suspect. The suspect may have cut himself going in or out of a window. When that happens, be sure to collect samples of the blood and samples of the glass found at the scene. Later, you can match the suspect’s blood to what you found at the scene. You may also find glass from the broken window embedded in the suspect’s shoe or boot. If you have the glass sample from the scene, you can make a match.

Often you’ll find other types of trace evidence at the scene which you can tie to the suspect. Examples include hairs, various kinds of fibers, plant matter, soils, etc. Collect this evidence from clothing, bedding, carpets, upholstered items such as chairs and sofas, and the interiors of cars, etc. Use a vacuum to collect the evidence from carpets, upholstery, and cars. Look for the same evidence on the suspect. If you apprehend a suspect relatively quickly after the crime occurred, you may find trace evidence from the victim on the suspect’s clothing. Alternatively, you may be able to match fibers from his clothing or his hair to trace evidence found at the scene. This kind of evidence may prove crucial to your case.

From: What Can We Get From a Suspect? by Dick Warrington in Who Says You Can't Do That