Amy Davidson writes in the New Yorker about the young injured Saudi man who was called a suspect in the Boston bombings by some US media outlets like the New York Post:
What made them suspect him? He was running—so was everyone. The bystander handed the man to the police, who 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?
It was reported later that he was a witness, not a suspect.