Well, where did you get these files from(I'm assuming through the Gnutella network, but I don't want to make an assumption)? If you were mass downloading, and never deleted any of the files you downloaded, Its extremely possible you could have downloaded these files and had been sharing them for quite some time. If you were using Limewire, It comes standard to share every file you download (unless you specifically tell it not too, most people don't know this). Alot of people have come across this stuff before using p2p, but delete it immediately, and for the most part, end up being fine. However, if you download this stuff, accidentaly or otherwise, and don't delete it, it would give authorites the right to believe that you were sharing these files on purpose, thus giving them the reason to go after you and confiscate your computer. Word to the wise, always check every file you download.

Did they by chance tell you how long they were monitoring those files you had on your computer?