The RIAA says the username of another computer user was found in the header of one of the MP3s. Headers are routinely used to store a song's title and length, but some MP3 compression software may also add information such as the username of the person who created the file
http://www.newscientist.com/news/news.jsp?id=ns99994111

I didn't think that RIAA people were intelligent enough to know what a header was.
But seriously, you are not anonymous.