A friend of mine recommended Limewire several months ago. I've gotten a lot of material off this great file-sharing service, but only audio and only mp3s, and nothing suspicious or shared by less than 3 users.

Today I turn on my computer and I get this message through email:

The US Government has been monitoring your computer for the past 24 hours and has noticed a substantial amount of pirating. You have 48 hours to delete the material from your computer and to destroy any hard copies made. If you should choose to ignore this warning you will be fined. Fines vary by song and range anywhere from $150,000 to $300,000.

I was terrified. I was led to believe that Limewire was completely legal. What's going on?

I've deleted 100% of the files I've downloaded after this - no exceptions. I just need to know how this could happen on a legal file-sharing program.