The message said in part: "It appears that you are offering copyrighted music to others from your computer. ...When you break the law, you risk legal penalties. There is a simple way to avoid that risk: DON'T STEAL MUSIC either by offering it to others to copy or downloading it on a 'file-sharing' system like this. When you offer music on these systems, you are not anonymous and you can easily be identified."

The mass messaging came after a federal judge on Friday delivered a setback to the music industry's efforts to shut down song-swapping services, and a day after Apple Computer Inc. unveiled an online music store aimed at wooing users from the free networks.

The RIAA said that by using song titles, it was identifying users who were posting copyrighted songs for others to download as targets for the messages, which were sent through the peer-to-peer networks' own systems.