Much can be learned from going over his cash and credit card receipts and by questioning the people around him. Find out what movies he was renting, what he was reading, who he was talking to and what was said, what and where he was eating (would establish if he was adhering to an Islam diet), what kind medication he was on (if any).
The issue here is actually quite simple. If it was a simple suicide, fuggedaboutit. But, if his motive turns out to be Islamic (even if he had second thoughts or just failed in his attempt to get into the stadium), there is a case and there are most likely guilty co-conspirators, who, if not stopped, might strike again.
the fact is he tried to buy amoniam nitrate 2 days before. Generally suicidal people do not buy farm products since they see themselves dead before the rainy season. It is amazing how quickly the FBI can reassure us there is no plot or terrorism.