OK, let's do this once and for all - who is the Ass Crack Bandit? I heard a theory that it's the dean. However, I don't think so. Annie said that ACB did something in 10 minutes while Annie only managed to do in 20 minutes. The dean isn't very fast, and as we found out in the last episode, the dean isn't allowed in the faculty lounge, so he couldn't have taken a shortcut. Another theory I heard is that ACB is not one person, but two - it's what explains the thing Annie said (the one I talked about earlier). So who are those people? Here's a theory I found convincing: Troy and Abed are ACB. Abed is the fastest guy in the school, so he could have done what Annie said. It also suits what he says when the dean asks him to stand in the crime scene and say what he sees. More than that, it fits Abed's character. He saw that there weren't any great episodes in a while, because of the gas-leak year and all, so he wanted to create some action. Troy is also very fast, so even if Abed couldn't have done what Annie said, both of them could do it. Also, the letter at the beginning sounds a lot like Troy wrote it. But wait, you might be asking, Troy got cracked! Well, there's a very simple explanation to this. Troy likes butt stuff. Edit: after watching the last episode of the show, I was thinking - could it be Annie? In the last episode, when Jeff brings up the ACB issue, Annie says, and quite suspiciously so, "oh yeah... Haha. Could've been anyone, right? Anyone." Any - one. Annie - one. LOL no I'm just playing with it. But now for real - Annie acted like it was her in the last episode. This got me suspicious. Remember that thing Annie said in the ACB episode? About the thing ACB did in 10 minutes and Annie could only do in 20? Well, maybe she was lying? And another thing, at the end of the episode, Jeff says that maybe they'll never figure out who he (ACB) is. Annie says, "or she." And then immediately looks at Jeff with a certain realization - but maybe it wasn't realization, but fear - she thought that this could help Jeff figure out that it's actually her? I'm just throwing it here, I'm not even sure it makes sense. But it's fun thinking about. Edit-Edit: now I'm thinking about it again, and maybe it's possible that it's not Annie, but Abed & Troy as I said earlier - Annie's reaction in the last episode could mean not that it's her, but that Abed told her it was him so she wants to protect him. We've seen how close they've gotten in the last season, so that makes a lot of sense.