Crime Library: Criminal Minds and Methods

LA Forensics: The Signature Murders

Print Comparisons

As SID collected fingerprint samples from every surface that might have been relevant to the crime, such as the knife, the desk, and the dresser drawers, they knew that it would all go into a database. To compare them for identification purposes, the print technician or detective must first make sure that prints are taken of everyone who was or who might have been at the scene, including corpses. To take a print, an ink roller is run over the fingertips and the tips are then pressed against a card with ten separate spaces, or a person places his or her fingertips against an ink pad and then presses the card.

Expert Compares Prints
Expert Compares Prints

Since 1972, fingerprints have been compared and retrieved via computer. By 1989, they could be sent to other places online. State and local agencies built up automated fingerprint identification systems (AFIS), and the FBI opened the National Crime Information Center (NCIC), which expedited the exchange of information among law enforcement agencies. They introduced a standard system of fingerprint classification (FPC), so that information could be uniformly transmitted from one AFIS computer to another.

The computer scans and digitally encodes prints into geometric patterns. In less than a second, the computer can compare a set of ten prints against a half million (although getting matches can take longer). At the end of the process, it comes up with a list of prints that closely match the exemplars (the originals). Then the technicians make the final determination via a point-by-point visual comparison.

At the Garcia scene, many items had the potential for prints, and the technicians collected quite a few, but only seven were of sufficient quality to put through the AFIS system. Yet fingerprints weren't the only evidence collected from this scene. At the time, SID was using a new technology.

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