Amy Davidson writes about a particularly quick judgment for The New Yorker:

What made them suspect him? He was running—so was everyone. The police reportedly thought he smelled like explosives; his wounds might have suggested why. He said something about thinking there would be a second bomb—as there was, and often is, to target responders. If that was the reason he gave for running, it was a sensible one. He asked if anyone was dead—a question people were screaming. And he was from Saudi Arabia, which is around where the logic stops. Was it just the way he looked, or did he, in the chaos, maybe call for God with a name that someone found strange?