The music industry's methods of tracking down suspected music pirates have been revealed for the first time.
Investigators say they can track files back to illegal websites
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.
In court papers, the RIAA said it could use the hashes to tell whether a file was recorded from a legitimately-bought CD or whether it was downloaded from the internet.
They can be used to track songs that were downloaded using the Napster service as far back as May 2000, the papers said.
Hashes are also used by the FBI in hacking investigations.
The RIAA has accused Nycfashiongirl of offering more than 900 songs by artists like the Rolling Stones, U2 and Michael Jackson for illegal download.
The songs had been traced back to Napster using the digital investigation techniques, the RIAA said.
Likening her to a shoplifter, the association said she was "not an innocent or accidental infringer".
"The source for Nycfashiongirl's sound recordings was not her own personal CDs," the RIAA's lawyers wrote.
But her lawyers said the recordings came from CDs she had bought, and the issue was "a smokescreen" to divert attention from the issue of whether she should be identified.
The RIAA has filed more than 1,300 subpoenas to force internet service providers to reveal the identities of their users.
It expects to file several hundred lawsuits, seeking damages of between $750 (£500) and $150,000 (£95,000) for each song stored illegally on a computer, as soon as next month.