If you're already yelling out "Britta!" or "Annie!" like many of the internet's pop culture sleuths, well son, you're wrong. Annie's handwriting doesn't match that of the Bandit's letter at all, and Britta is too short. Yes, it all seems to come down to height, as these things often do. Or not, we're no Sherlocks. Anyway, there's a shot of the ACB in the episode, hiding in the background right before Troy gets coined in the crack. It's the only shot we ever see of the Bandit, and it shows a tall, lanky figure ...Â
 ... with the same height as Abed. Those school shelves are typically 7 feet tall, and the Theorists calculated that this shot shows the Bandit to be anywhere between 5'10 and 6 feet tall. Britta is only 5'4, and not really known for walking around in stilettos. Abed though? Yeah, he's 6 feet tall.Â
There's also a blink-and-you'll-miss-it shot of Abed in the gym hall:Â
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Seconds later, as the Dean addresses the Greendale students, he's gone ...Â
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... and the ACB strikes moments later, dropping a coin down a crack from underneath the bleachers.
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Additional damning evidence the investigation doesn't bring up is that the phone call between the Dean and the ACB suspiciously resembles Abed's speech patterns. Given the fact that Abed often goes to great lengths to make sure his fictional narrative plays out exactly the way he wants it to, there's really not much doubt left in the end. He's also the most likely to get away with it, anyway.