Seconds later, the first explosion went off about a block down the street and spread fear and confusion through the crowd. But Tsarnaev — unlike nearly everyone around him — looked calm and quickly walked away, the FBI said.
Just 10 seconds or so later, the second blast occurred where he had left the knapsack, the FBI said.
The FBI did not make it clear whether authorities believe he used his cellphone to detonate one or both of the bombs or whether he was talking to someone.
The court papers also said that during the long night of crime Thursday and Friday that led to the older brother's death and the younger one's capture, one of the Tsarnaev brothers told a carjacking victim: "Did you hear about the Boston explosion? I did that."
The brothers are ethnic Chechens from Russia who have lived in the U.S. for about a decade. Investigators are focusing on a trip the older brother made last year to Chechnya and Dagestan, in a region of Russia that has become a hotbed of separatist politics and Islamic extremism.
In addition to the federal charges, the younger Tsarnaev brother is also likely to face state charges in connection with the shooting death of an MIT police officer.
The Obama administration said it had no choice but to prosecute 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 U.S. 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. Carney said that since 9/11, the federal court system has been used to convict and imprison hundreds of terrorists.