The Recording Industry Association of America (RIAA) is continuing its tactic of filing suits against anonymous John Doe computer users who the organisation accuses of illegally sharing copyright material over peer-to-peer networks.
The association filed lawsuits against 531 unnamed people on Tuesday, claiming that they are offering substantial amounts of copyrighted music files for free.
The legal actions were filed in Philadelphia, Atlanta, Orlando and Trenton, New Jersey, the RIAA said.
The RIAA began the John Doe lawsuits, in which users are identified only by their Internet Protocol (IP) addresses, last month when it filed suit against 532 other anonymous users.
Previous attempts to subpoena Internet service providers (ISPs) directly, without court approval, for the names and addresses of users had failed when the US Court of Appeals for the District of Columbia overturned a lower court ruling that allowed the practice.
San Francisco-based privacy group, the Electronic Frontier Foundation (EFF), accused the RIAA of cutting corners with the lawsuits.
The record industry had failed to follow the basic rules of filing lawsuits, by lumping groups of people into each suit, the EFF said.
The people involved were in different parts of the US, use different software and were allegedly involved in sharing a wide variety of music, it said.
The accused had also not been given any way of reviewing and responding to the accusations before their identities were revealed, the EFF said.