They released the guy because he promised the FBI that he didn't know what was in his roommate's room. He probably said the door was always locked and he wondered himself what was going on. He probably said that the strange beard the guy grew was because he was a wannabe, but didn't ever fit in with the Muslim student crowd. In fact, it is possible this kid was just plain crazy. And the FBI knows this from his psych. history. The reason I am saying this is that the feed store man reports the kid was muttering to himself. He sounds like he had mental problems.
I would think that 19 hijackers had mental problems too. Anyone who kills themselves in the name of Allah is NUTS. It still doesn't stop them from being TERRORISTS or wannabees.