Tsarnaev is being held in a Boston hospital.
His brother, Tamerlan Tsarnaev, was killed after a firefight with police.
Officials also told the AP on Wednesday that the homemade bombs used to kill three people and injure more than 200 were triggered by a remote detonator of the kind used in remote-control toys.
Investigators found pieces of the remote-control equipment among the debris, the officials said.
"That’s the igniter," Rep. Michael McCaul (R-Texas), chairman of the House Homeland Security Committee, said in an interview with Bloomberg. "When he turns on the device, it triggers the ignition and it blows up."
McCaul said the perpetrators "had some level of training," based on the level of sophistication in the bombs.