You do know that when you torrent, you do have access to IPs of concurrently leeching the content. So, these enforcers are there hiding/leeching/tracking/seeding and they get you, and when they get you, they through your ISP send you letters like that. If it's not your fault, you've every right to defend yourself but if it's yours, I haven't yet forgotten a case of a woman who downloaded songs and shared it, only for her to be in the jail for over 50 years and penalties of $200,000. Defendants can't really do when these big guys come after you.

If I were you,  I would talk to your ISP and see what they say or resolve the case ASAP.