How Crime Scene Analysts Collect and Preserve Fingerprints

From Mark Twain to CSI, identifying fingerprints is among the most well-known methods for solving crimes in fiction. In real life, collecting fingerprints left behind on a window or door knob is a delicate science, as well as an important one. Preserving fingerprints can help definitively link a person to a crime, especially since the US government already maintains biometric databases of fingerprints from millions of people, including previously convicted and suspected criminals, government employees, military personnel, and other groups.

To learn more about the science of fingerprint collection, WIRED caught up with Matthew Steiner, a certified senior crime scene analyst with more than 20 years of experience. He demonstrated how to lift fingerprints off of different surfaces, so they can be viewed under a microscope.

“The first thing we want to consider is the surface itself,” Steiner says. “The most ideal surface for fingerprints is something that’s smooth and nonporous,” like plexiglass. Steiner uses a special white powder to gently dust fingerprints, making them visible to the human eye. Tape is then placed over the powder, to lift prints off the surface and preserve them for later analysis.

But what if fingerprints are on a surface that isn’t flat, like a door handle? In those cases, Steiner makes a silicone cast of the item, which is then sent to a lab. Steiner can even lift fingerprints from items that have been rained on. “Fingerprints are comprised mostly of water,” he says, but also a small percentage of minerals, salts, amino acids, and lipids. If an object has gotten wet, Steiner can spray it with a special black fingerprint detection chemical. It reacts with the remaining oils, rendering the print visible.

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