HOW DOES IT WORK?
In a forensic investigation, the investigator is trying to build a profile of you. The first piece of evidence he will look for is your footprint. You scatter these behind you in abundance. Every time you walk into a room with mud or dust on the bottom of your shoe, every time you cross your lawn, and every time you step in the blood of your victims, you leave a footprint behind you. An analyst can discover a wealth of information about you in the swoops and curves of your shoe bottom.
Computerized databases instantly match the make and model of a shoe. Your weight, height, and the way you walk become public knowledge. The spacing and smudging of footprints indicate your mental state—are they erratic and repetitive, indicating panic? Or are they the measured steps of perfect calm? Their placement helps create a narrative of the sequence of events. Dirt shed from the bottom of your shoe is used to determine where you came from. If it occurs to you to ditch your footwear, don’t. Even more information can be gleaned by looking at the impression your foot makes inside your shoe. Thinking of trying to hide the evidence of your passage by sticking strictly to carpet? Don’t bother. Electrostatic dust and ultraviolet light lift your footprints from a carpet the same way they lift fingerprints from glass.
Speaking of fingerprints, that’s the next thing someone trying to profile you will look for. Your fingertips produce an oil you leave behind on every object you touch. Usually, the fingerprint becomes smudged as your fingers drag slightly across a surface, but sometimes a clear picture of one or more fingers is left behind. If you’ve got a touchscreen phone, look at it right now. See the streaks on it? Finger grease. That stuff gets everywhere.
It’s true that no two fingerprints are the same, but that includes prints taken from the same finger on the same hand only moments apart. Skin is flexible, which means your fingertip distorts slightly every time you press it to a surface. The identification of a fingerprint is more art than science, reliant on the guesswork of skilled individuals and expert computer systems, and with a false positive rate as high as 20 percent, giving you a solid one-in-five chance of being fingered as a serial killer.
Fingerprints captured in the wild are called latent prints. When an object is found that is likely to have a usable print on it, the surface is covered with either a fine dust or chemical agent that reacts with the sweat in the prints. This increases the contrast between the surface and the print, rendering the print visible. It can then be photographed or lifted on a thin plastic film for later analysis.