After the first blast, a block away from Dzhokhar, "virtually every head turns to the east ... and stares in that direction in apparent bewilderment and alarm," the complaint says. But Dzhokhar Tsarnaev, unlike practically everyone else around him, appeared calm, the FBI said.
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.
Sullivan reported from Washington. Contributing to this report were Associated Press writers Steve Peoples, Allen Breed, Bridget Murphy, Jay Lindsay and Bob Salsberg in Boston and Pete Yost in Washington.
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