Dzhokhar Tsarnaev remains in a serious condition, two days after being pulled bloody and wounded from a tarp-covered boat in a Watertown backyard.
The capture came at the end of a tense Friday that began with his 26-year-old brother, Tamerlan, dying in a gun battle with police.
There was no immediate word on when Tsarnaev might be charged and what those charges would be.
The twin bombings last Monday killed three people and wounded more than 180.
The most serious charge available to federal prosecutors would be the use of a weapon of mass destruction to kill people, which carries a possible death sentence. Massachusetts does not have the death penalty.
Massachusetts governor Deval Patrick said that surveillance video from last Monday’s Boston Marathon attack shows Dzhokhar Tsarnaev dropping his backpack and calmly walking away from it before the bomb inside it exploded.
“It does seem to be pretty clear this suspect took the backpack off, put it down, did not react when the first explosion went off and then moved away from the backpack in time for the second explosion,” Mr Patrick told NBC television.
“It’s pretty clear about his involvement and pretty chilling, frankly.”
He added that he has not viewed all the tapes but had been briefed by law enforcement about them.
Meanwhile, the Boston police commissioner said yesterday investigators believe the two suspects were likely to have been planning other attacks, based on the weapons uncovered.
Ed Davis told CBS’s Face The Nation programme that authorities found an arsenal of homemade explosives after Friday’s gun battle.
“We have reason to believe, based upon the evidence that was found at that scene — the explosions, the explosive ordnance that was unexploded and the firepower they had — that they were going to attack other individuals,” Mr Davis said. “That’s my belief at this point.”
The scene of the gun battle was loaded with unexploded bombs, and authorities had to alert arriving officers to them and clear the scene, Mr Davis said. One improvised explosive device was found in the Mercedes the brothers are accused of carjacking, he said.
“This was as dangerous as it gets in urban policing,” he added.
Mayor Patrick told Face The Nation he has no idea what motivated the suspects. The governor said it is hard to imagine why someone would deliberately harm “innocent men, women and children in the way that these two fellows did”.