Using digital fingerprints, or "hashes", investigators say they can tell if an MP3 file was downloaded from an unauthorised service.

The industry also tracks "metadata" tags, which provide hidden clues about how files were created.

The details were given by the Recording Industry Association of America (RIAA) in a legal case against a suspected pirate.

The association is trying to force a woman from Brooklyn, New York, who is accused of distributing almost 1,000 songs over the internet, to reveal her real identity.

She is currently only known by her screen name, "Nycfashiongirl", and wants to remain anonymous.

[ Read more ]