CNN.com – Revealed: How RIAA tracks downloaders – Aug. 28, 2003

For example, the industry disclosed its use of a library of digital fingerprints, called “hashes,” that it said can uniquely identify MP3 music files that had been traded on the Napster service as far back as May 2000. Examining hashes is commonly used by the FBI and other computer investigators in hacker cases.
By comparing the fingerprints of music files on a person’s computer against its library, the RIAA believes it can determine in some cases whether someone recorded a song from a legally purchased CD or downloaded it from someone else over the Internet.