Investigators have used fingerprints to identify criminals for over a century. While other visible characteristics may change over time, fingerprints do not. Fingerprints are the residue of perspiration secreted along the tops of “friction ridges” on fingertips. When people touch something, they leave what is called a “latent” print. Latent prints are not usually visible to the naked eye.

Analysts reveal latent prints by using powders, special lighting, or chemicals, depending on the surface on which the print was left. They look at specific areas of prints for points that match between a suspect’s fingerprint and the latent print from the crime scene. There is no international standard for the number of points that result in an identification match. Postal Inspection Service technicians look for seven to nine points before declaring a match.