Fingerprint evidence rests on two basic principles:

* A person's "friction ridge patterns" (the swirled skin on their fingertips) don't change.
* No two people have the same pattern of friction ridges.

Police officers can use fingerprints to identify defendants and crime vict...

Fingerprint evidence rests on two basic principles:

* A person's "friction ridge patterns" (the swirled skin on their fingertips) don't change.
* No two people have the same pattern of friction ridges.

Police officers can use fingerprints to identify defendants and crime victims if a print matches one already on file. (Today, the FBI has a collection of prints that numbers in the millions.) People's fingerprints can be on file for a variety of reasons. For example, people may be fingerprinted when they are arrested, or when they begin certain occupations. And it is increasingly popular for parents to ask local police departments or schools to fingerprint their young children, a grim reminder that children who are abducted or are the victims of other heinous crimes often cannot be identified otherwise.
How Fingerprints Are Found

Friction ridges contain rows of sweat pores, and sweat mixed with other body oils and dirt produces fingerprints on smooth surfaces. Fingerprint experts use powders and chemicals to make such prints visible. The visibility of a set of prints depends on the surface from which they're lifted; however, with the help of computer enhancement techniques that can extrapolate a complete pattern from mere fragments, and laser technology that can read otherwise invisible markings, fingerprint experts increasingly can retrieve identifiable prints from most surfaces.

The age of a set of fingerprints is almost impossible to determine. Therefore, defendants often try to explain away evidence that their fingerprints were found at crime scenes by testifying that they were at the scene and left the prints at a time other than the time of a crime.
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