Something tells me if OP openly asked their roommates “who dunnit” and no one spoke up, the culprit probably immediately deleted all the files and traces of it.
OP, I have another random thing to try - this might sound like a very rudimentary way to do things but honestly just get everyone together in the same room without announcing it beforehand, then collectively make the decision to check each person’s computers specifically for torrenting software or even the film in question.
I suspect the culprit already deleted their stuff but if they didn’t already, more than likely this announcement will cause there to be 5 people happy to oblige and 1 very suddenly-privacy-conscious person objecting. There’s your main suspect. This obviously isn’t legal advice, just a fun little thing to try. You might have better luck getting an answer from your ISP.