Probably because of the questions the police were asking during their "visit" to Edens dorm. The police were reported to be asking the other students in the dorm questions regarding whether certain people were or were not at the party.
The police were probably focusing their question on one or two names (the guy the AV is "90% sure of") and that's how the Edens folk got the idea of who it is.
I'm a little confused. I know that the police were asking "who" was at the party. But, I thought the players did not talk to the police? Did they just not answer the questions?