I always assumed that Dean meant the maniac to be Crouch Jr. but there is nothing that clearly states the truth on that. It certainly wouldn't come from the ministry, who probably wouldn't want to announce to the world that a death eater had a hand in what took place at Hogwarts because it would give credibility to Dumbledore saying that Voldemort is back. So the only person who might have told them would have been Dumbledore at the end of term. In the chapter before Seamus asks Harry what happened that night...so Dumbledore obviously didn't give all the details. So now that you mention that, I doubt he did tell them who Prof. Moody was all semester. They probably just think the old guy finally lost it. It's an interesting argument.