The bombs were triggered by a remote detonator of the kind used in remote-control toys, U.S. officials said Wednesday. They said investigators found pieces of the remote-control equipment among the debris and were analyzing them.
Both U.S. officials are close to the ongoing investigation but spoke on condition of anonymity because they were not authorized to discuss it publicly. One official described the detonator as "close-controlled," meaning it had to be triggered within several blocks of the bombs.
The criminal complaint filed against Dzhokhar Tsarnaev said he was using a cellphone moments before the blasts.
U.S. officials also said he has told interrogators he and his brother were angry about the U.S. wars in Muslim Afghanistan and Iraq.
After closed-door briefings on Capitol Hill with the FBI and other law enforcement officials, lawmakers said earlier this week that it appeared so far that the brothers were radicalized via the Internet instead of by direct contact with any terrorist groups and that the older brother was the driving force in the bomb plot.
In Russia, U.S. investigators traveled to the predominantly Muslim province of Dagestan and were in contact with the brothers' parents, hoping to gain more information.
The parents, Anzor Tsarnaev and Zubeidat Tsarnaeva, plan to fly to the U.S. on Thursday, the father was quoted as telling the Russian state news agency RIA Novosti. The family has said it wants to bring Tamerlan Tsarnaev's body back to Russia.
Investigators are looking into whether Tamerlan, who spent six months in Russia's turbulent Caucasus region in 2012, was influenced by the religious extremists who have waged an insurgency against Russian forces in the area for years. The brothers have roots in Dagestan and neighboring Chechnya but had lived in the U.S. for about a decade.