What’s in a print? Crime scene investigation and fingerprints

March 27, 2017

Three images of fingerprints.
Photo: Most fingerprints fall into the categories of arches, loops, and whorls, and they’re critically important in investigating crimes.​


They’re right up there with snowflakes for uniqueness, only they don’t melt and you have them your whole life: fingerprints. And if you’ve watched even one crime movie or cop show, you know how important fingerprints are in crime scene investigation. In fact, they’re one of the most common types of evidence investigators search for.

Although every fingerprint is different, they’re all variations on three broad categories: the arch, which looks a bit like a cross-section of a hill; the loop, which is teardrop-shaped; and the whorl, which is reminiscent of a whirlpool.

There are also three categories of prints that can be gathered from a crime scene: patent, plastic, and latent. A patent print is what’s left when you have liquid on your fingers—ink or blood, for example—and touch a smooth surface. It’s visible to the naked eye.

Plastic prints are essentially impressions made when you touch something soft and malleable like wax or fresh paint. You might have studied your own when playing with Play-Doh or silly putty as a kid. They’re also visible to the naked eye.

Latent prints are invisible but very common. We leave them everywhere: computers, phones, doorknobs, lamps, steering wheels. There are various ways to process latent prints to make them visible, such as lasers, powders (hence the term “dusting for prints”) and different light sources.

The Minnesota Bureau of Criminal Apprehension (BCA) has a Latent Print Section that processes fingerprints on evidence from law enforcement agencies all over the state. And when a medical examiner’s office receives the body of an unknown person, the Latent Print Section works with them to determine the identity using fingerprints.

The BCA also manages the Midwest Automated Fingerprint Identification Network (MAFIN), which is a database of fingerprint cards from arrest records from Minnesota, North Dakota, and South Dakota. They use MAFIN to compare unidentified fingerprints from crime scenes, and a latent print examiner reviews the list of possible matches.

So next time you’re drinking from a water glass or have ink on your fingers, take a look at your fingerprints. Not only are they part of what makes you unique, they are a critical tool in solving crimes.