In forensics, latent fingerprints are marks left at the scene of a crime that may not be immediately visible to the naked eye. To expose them, technicians use fingerprint powder, fuming, and other techniques. Identifying these fingerprints is an important part of evidence collection, and many technicians specialize in retrieving fingerprints from crime scenes and analyzing them in the lab in order to assist law enforcement with apprehending criminals.
The hands and feet are covered in a natural secretion due to the eccrine glands, which produce sweat, a mixture of water, salts, and other trace compounds. The sweat adheres to the friction ridges of the finger, and when a finger is placed on a surface such as glass, plastic, or wood, an impression will be left behind. The natural secretions of the body preserve the fingerprint, which is utterly distinct — no two humans have the same fingerprints.
Latent fingerprints are usually left behind by accident, because a careless criminal did not realize that his or her hands might leave a mark. Sometimes, fingerprints are made from substances other than sweat, such as blood, other body fluids, or paint. In this case, they are called patent fingerprints. Part of the analysis performed on fingerprints includes a determination of what the print was made from, as this may provide additional evidence about the criminal or the crime.
When investigators arrive at a crime scene, one of the first things they do after photographing the entire scene is dust for latent fingerprints. This ensures that no prints are missed, even if patent fingerprints, prints readily visible to the naked eye, are evident. Specialized fingerprint powder is gently brushed over surfaces which may hold fingerprints to see if any appear. If fingerprints emerge after dusting, they are photographed and then carefully lifted with clear tape before being affixed to cards. For difficult surfaces, another process known as fuming may be used to find these marks. Fuming can be used for very old fingerprints, because it causes a chemical reaction with trace substances that may be left behind, even if the sweat itself is gone.
The fingerprints are brought back to a crime lab for analysis, which usually starts with scanning them into a computer and cross checking them against a criminal database for matches. In the United States, the Integrated Automatic Fingerprint Identification System (IAFIS) is a national database that police investigators can use to get more information about prints found at crime scenes. The electronic database contains over 47 million records and can be used to instantly check fingerprints from a crime scene. Fingerprint records for IAFIS come from criminals, as well as citizens who are fingerprinted as part of a routine background check, and the substantial database greatly assists law enforcement in their jobs.