He confessed. Even if he had not, I'm sure they could have run forensics on his laptop to find traces of the email. All they needed was probable cause to narrow

He confessed. Even if he had not, I'm sure they could have run forensics on his laptop to find traces of the email. All they needed was probable cause to narrow the investigation, which they got. You also seem to be under the impression that circumstantial evidence would not be enough to convict -- it often is, when combined with motive. I'm guessing the suspect had finals in those buildings that day, and was under considerable stress.