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. Dzhokhar Tsarnaev, "virtually alone of the individuals in front of the restaurant, appears calm." He then quickly walked away, leaving a knapsack on the ground, officials said. 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.
The criminal complaint shed no light on the motive for the attack.
The Obama administration said it had no choice but to prosecute Dzhokhar Tsarnaev in the federal court system. Some politicians had suggested he be tried as an enemy combatant in front of a military tribunal, where defendants are denied some of the usual constitutional protections.
But Tsarnaev is a naturalized U.S. citizen, and under U.S. law, American citizens cannot be tried by military tribunals, White House spokesman Jay Carney said. He said that since the Sept. 11 terrorist attacks, the federal court system has been used to convict and imprison hundreds of terrorists.
Also on Monday, Kazakhstan's Ministry of Foreign Affairs issued a statement saying two foreign nationals arrested Saturday in the Boston area on immigration violations are from Kazakhstan and may have known the two Marathon bombing suspects. The foreign ministry said U.S. authorities came across them while searching for "possible links and contacts" to Dzhokhar Tsarnaev. Their names have not been released.
Shortly after the charges against Tsarnaev were unveiled, Boston-area residents and many of their well-wishers — including President Barack Obama at the White House — observed a moment of silence at 2:49 p.m. — the time a week earlier when the bombs exploded.