The brothers decided to carry out the attack before Independence Day when they finished assembling the bombs, the surviving suspect told interrogators after he was arrested, according to two U.S. officials briefed on the investigation. The officials spoke on condition of anonymity because they were not authorized to discuss the ongoing investigation.
Investigators believe some of the explosives used in the attack were assembled in Tamerlan Tsarnaev's home, though there may have been some assembly elsewhere, one of the officials said. It does not appear that the brothers ever had big, definitive plans, the official said.
The brothers' mother insists the allegations against them are lies.
Meanwhile, the Department of Homeland Security ordered border agents to immediately begin verifying that every international student who arrives in the U.S. has a valid student visa, according to an internal memorandum obtained Friday by The Associated Press. The new procedure is the government's first security change directly related to the Boston bombings.
The order from a senior official at U.S. Customs and Border Protection, David J. Murphy, was circulated Thursday and came one day after President Barack Obama's administration acknowledged that one of the students accused of hiding evidence, Azamat Tazhayakov, of Kazakhstan, was allowed to return to the U.S. in January without a valid student visa.
Tazhayakov's lawyer has said he had nothing to do with the bombing and was shocked by it.
A benefit concert featuring Aerosmith, James Taylor and Jimmy Buffett is scheduled for May 30 at the TD Garden in Boston. The proceeds will go to The One Fund, which has taken in more than $28 million for those injured and the families of those who were killed.
The fund's administrator, Kenneth Feinberg, said Friday he plans to hold meetings with victims next week and begin cutting checks by the end of June.