A top US interrogation group is waiting to question the surviving suspect in the Boston Marathon bombings.
Dzhokhar Tsarnaev, 19, was arrested late on Friday when he was found seriously injured in a suburban backyard after a huge manhunt.
He is under armed guard in hospital. Massachusetts Governor Deval Patrick said the suspect was stable, but not yet able to communicate.
The teenager's brother, Tamerlan, died after a shoot-out with police.
Three people were killed and more than 170 others injured by Monday's twin bombing, close to the finish line of the Boston Marathon.
Police officer Sean Collier was shot dead during the police operation to find the brothers on Thursday night. A transport officer was later seriously injured in the shoot-out which left Tamerlan Tsarnaev fatally wounded.
On Saturday, President Barack Obama met his top security advisors to review the events in Boston.
He has vowed to seek answers on what motivated the alleged bombers and whether they had help.
'A million questions'
Dzhokhar Tsarnaev was found by a member of the public on Friday evening, shortly after a city-wide lockdown was ended. He was injured and hiding in a boat in a backyard, and was reportedly further injured in a fire fight with police.
The High-Value Detainee Interrogation Group - a multi-security agency unit specialising in questioning terror suspects - is waiting at the Beth Israel Deaconess Medical Center in Boston as he recovers.
The BBC's David Willis, outside the hospital, says the suspect is suffering gunshot wounds to the neck and leg and has lost a lot of blood, so it could be a while before investigators are able to talk to him.
CBS News quoted investigators as saying that a wound to the neck may have been a suicide attempt.
Prosecutors are also at the scene, determining what charges the teenager might eventually face.
A federal charge of using a weapon of mass destruction to kill people carries a possible death sentence. There is no death penalty in the state of Massachusetts.
On Saturday, Governor Patrick told reporters: "I, and I think all of the law enforcement professionals, are hoping for a host of reasons that the suspect survives because we have a million questions, and those questions need to be answered."
In a move criticised by rights activists, officials have said they intend to question the teenager without reading him his Miranda rights - the standard statement informing suspects they have a right to a lawyer and to remain silent - citing a "public safety exception".
The American Civil Liberties Union said such an exemption only applied in the case of immediate threats, and that the suspension of rights could not be "open-ended".
"Denial of rights is un-American and will only make it harder to obtain fair convictions," it said in a statement.
But Republican Senators John McCain of Arizona and Lindsey Graham of South Carolina say Dzhokhar Tsarnaev should be treated as an "enemy combatant", meaning he would not be entitled to the same legal rights as a criminal defendant.
"We should be focused on gathering intelligence from this suspect right now that can help our nation understand how this attack occurred and what may follow in the future," their statement read.
One key line of inquiry will be a six-month trip made by Tamerlan Tsarnaev to Dagestan in the Russian Caucasus in 2012.
The news that one suspect had been killed and the second captured prompted scenes of celebration on the streets of Boston on Friday evening, with people cheering, honking car horns and waving American flags.
Elliot Friar, who lives close to where Monday's bombs exploded, said it was "a bittersweet moment" because of those who had lost their lives.
"But it was also a time for celebration because the city has been on edge and we're finally feeling more safe than we have in the past four days," he told the BBC.
The Boston Red Sox baseball team had been due to play in the city on Friday, but the game was postponed because of the lockdown.
The game went ahead on Saturday, with team members wearing the city's name on their shirts instead of the usual Red Sox and the stadium observing a moment of silence for the victims.
Law enforcement officials and family members have identified the Tsarnaev brothers as ethnic Chechens who had been living in America for about a decade.
The FBI had interviewed Tamerlan Tsarnaev in 2011 after a request from a foreign government, US law enforcements officials have confirmed.
But agents closed the case after finding no cause for concern.
Several members of the Tsarnaev family have condemned and disowned the brothers, but their parents have said that they could not have planned such an attack as they were being monitored by the FBI.
Their mother, Zubeidat Tsarnaeva, said she was "100% sure that this is set up", insisting in an interview with Russia Today that her sons had never had any involvement with terrorism.