What gave the FBI or anyone else any initial suspicion that he'd be a suspect?

Just watched this last night. Really good movie actually. I stayed awake the whole time despite it being late at night (I might have zoned out a little a few times though and missed small details but never fell asleep.) I'm just curious as to why the FBI instantly thought it was him and really looked at no other people as a suspect? I know they said he had a past where he was rough on some of the college kids while he was a security guard, but outside of that, what else would have redflagged them? He was a white guy, single, with a past centering around law enforcement, and that was the main rationale? It just doesn't add up to me.

They said he was "over zealous" when the pipe bomb was found in the bag, and the fact he was on the back side of the structure that shielded him from getting seriously hurt made it seem as though he knew right where to be is something I just don't buy. Seems like the FBI failed hugely with their security and they needed a quick suspect to prevent from looking incompetent, so they went with the easy target who was too polite and docile to fight back against them. Pretty sad story overall though. The poor guy died young as well, and still likely carried those feelings of being a scapegoat and being routinely beaten down by people who looked down their noses at him to his grave even though he was proven innocent.