[mmcd]

It just seems like an awfully strange coincidence that method he's using to deal with the cheaters is the exact same method police and other authority figures use to unravel conspiracies. From what I gathered from the original post on the matter, it seems that this e-mail went out very soon after the exam was administered. It is very unlikely that he looked over all of the exams and picked out similarities that were indicative of a bunch of students working together. In fact, I think the only way this would be feasible was if the exam was scantron, and a bunch of them came out exactly the same.

Additionally, in a situation like this, what he "knows" and what he can prove are two entirely different things. Unless the students were blatantly obvious about what they did, there's really nothing he can do about it unless someone that was involved confesses.