It remained unclear on Sunday when Dzhokhar Tsarnaev might be able to answer questions. The 19-year-old suspect suffered a gunshot wound to his neck, which according to some reports was self-inflicted, during his standoff with police on Friday night.
He is in "serious condition" at Beth Israel Deaconess Medical Center according to a brief statement by the FBI.
CNN quoted an official who had been briefed on the case as saying Tsarnaev was "intubated and sedated."
"We don't know if we'll ever be able to question the individual," said Boston Mayor Tom Menino on Sunday on the ABC news program This Week, though he did not elaborate.
Some U.S. news media, quoting anonymous sources, reported Sunday night that Tsarnaev was awake and responding to questions by writing, but this has not been officially confirmed.
Three marathon spectators, including an eight-year-old boy, died in a blast near the finish line of the race last Monday, while more than 170 others were injured.
Boston's police commissioner, meanwhile, said the scene of Friday's wild firefight in suburban Watertown suggested the suspects — Tsarnaev and his now dead older brother, Tamerlan Tsarnaev — were plotting further attacks.
Edward Davis told CBS's Face the Nation that the scene was littered with unexploded improvised explosive devices. He said he believed the suspects "were going to attack other individuals."
The stockpile was "as dangerous as it gets in urban policing," said Davis.
Massachusetts State Police released a video on Sunday purporting to show Tsarnaev's hiding place shortly before he was taken into custody on Friday.
The video, taken from a helicopter hovering over a residential area, shows thermal images of a covered boat containing a glowing white mass which police said was Tsarnaev.
The date and time code on the material indicates it was shot on Friday at 7:22 p.m. ET.
Following the Boston bombing, in which two explosive devices were detonated, authorities said Dzhokhar Tsarnaev and his alleged co-conspirator and brother, Tamerlan, 26, killed a police officer near the Massachusetts Institute of Technology on Thursday night, before stealing a vehicle and heading to Watertown, to the east of Boston, where a police officer spotted them.
The brothers, one in the stolen car, the other in a second car, jumped out of the vehicles and started shooting at the lone officer even as backup rushed to the scene, said Watertown police officials.
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The suspects allegedly tossed grenades and Tamerlan Tsarnaev got within three metres of officers involved in the shootout before he ran out of ammunition and was tackled.
His injured younger brother fled, leading to an area lockdown with police and armed forces searching from house to house in Watertown.
Dzhokhar Tsarnaev evaded the army of search teams, and as night fell authorities scaled back the hunt.
Mass transit was allowed to resume, and people were told they could leave their homes.
Then, according to police a resident was checking the boat in his yard on Friday evening and noticed the straps weren't the way he left them.
He looked in the boat, saw blood and someone huddled in a corner and quickly called police.
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The discovery set up the final confrontation and led to the capture of the younger Tsarnaev brother.
Officers were taking no chances as they approached the boat.
In the video handout, an armoured vehicle is seen equipped with an arm which pulls back the tarpaulin covering the vessel.
Video shows flash-bang grenades used
Toward the end of the footage small explosions on the front of the boat are visible, believed to be flash-bang grenades used by the police to stun and disorient Tsarnaev.
Authorities said that they attempted to talk the suspect, already weakened by a gunshot wound received some 20 hours earlier, into getting out of the boat, but Tsarnaev exchanged gunfire with law enforcement officers for an hour before being captured and taken away to hospital in an ambulance.
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Prosecutors haven't said yet what charges Tsarnaev will face. The naturalized American citizen from Cambridge, Mass., could be charged with using a weapon of mass destruction to kill people.
"Charges will come soon and they will almost certainly be both federal and state-level charges — state murder charges and federal terrorism charges," CBC's David Commons reported from Boston on Sunday."
"We understand that those would have to be imminent. The FBI is expected to make an announcement at some point today, but we also expect the suspect will be appointed a federal public defender, and in a case like this, although it is extremely rare, the federal government could seek the death penalty."
Massachusetts does not have a state death penalty statute.