He then quickly walked away, leaving a knapsack on the ground; about 10 seconds later, a bomb blew up at the spot where he had been standing, the FBI said.
The FBI did not say whether he was using his cellphone to detonate one or both of the bombs or whether he was talking to someone.
Among the details in the affidavit:
— Dzhokhar Tsarnaev had gunshot wounds to the head, neck, legs and hands when he was captured hiding out in a boat in a backyard in the Boston suburb of Watertown, authorities said.
— One of the brothers — it wasn't clear which one — told a carjacking victim during their getaway attempt: "Did you hear about the Boston explosion? I did that."
— The FBI said it searched Tsarnaev's dorm room at the University of Massachusetts-Dartmouth on Sunday and found BBs as well as a white hat and dark jacket that look like those worn by one of the suspected bombers in the surveillance photos the FBI released a few days after the attack.