Ok, throwing in my two cents in this!
So we've been told it's someone we won't expect. Obvious choices - Tom Quill, Julie Quill, Paul, John Doe (he's a permanent character isn't he? Couldn't have him in jail) and Jacka - so based off of that we can assume it's not them? Did Jacka put the blueprints in Paul's briefcase or was that someone else? Having a memory block! haha
less obvious suspects - Nate? Apparently the guy who plays him is leaving, so it'd be a good exit, going to jail. He just so happens to reappear the day of the explosion after being gone for months, location unknown? He clearly doesn't like Paul. He's dead set against Aaron working with him, especially since it's to do with him potentially helping Paul find out the real culprit of the explosion and then he "accidentally" (well, presuming accidentally) knocks Paul down? Maybe he's somehow got involved with Jacka. I dno, he seems like an unlikely suspect, which in turn makes me suspect him
This is based off of pure speculation though, nothing concrete, so I could be very wrong!