Dzhokhar Tsarnaev, 19, was captured late on Friday after a gunfight with police that ended a daylong manhunt and sent waves of relief and jubilation throughout Boston.
His brother, Tamerlan, 26, died on early Friday after a shootout with police.
Dzhokhar had been hiding in a boat parked in the backyard of a house in the suburb of Watertown and was captured after a resident spotted blood on the boat and called police. He was being treated at Beth Israel Deaconess Medical Center in Boston.
Members of the FBI Evidence Recovery Team work around the home where Boston Marathon bombing suspect Dzhokhar Tsarnaev was hiding in Watertown. Reuters
Investigators are trying to establish whether the pair had assistance leading up to the detonation of bombs made in pressure cookers and packed with ball bearings and nails at the crowded finish line of Monday's marathon, killing three people and injuring 176.
Tamerlan, 26, traveled to Moscow in January 2012 and spent six months in the region, a law enforcement source said, but it was unclear what he did while he was there and if he could have had contact with militant Islamist groups in southern Russia's restive Caucasus region.
Early indications are the brothers acted alone, Watertown police chief Edward Deveau told CNN on Saturday.
"From what I know right now, these two acted together and alone," he said. "But as far as this little ... group, I think we got our guys." Bombers' path to Boston may have begun online
Still, the bombings prompted contact between the United States and Russia on terrorism, and the Kremlin said on Saturday the presidents of both countries agreed by telephone to increase cooperation on counterterrorism.
The FBI, who interviewed Tamerlan in 2011 after he was flagged by Russian authorities, believes the older brother was the leader of the pair, although investigators were checking on people who had contact with both brothers to see if anyone else was involved, a senior US law enforcement source said.
Ruslan Tsarni, who said he was an uncle of the brothers, told CNN on Saturday he first noticed a change in Tamerlan Tsarnaev's religious views in 2009.
Investigators work around the boat where Dzhokhar Tsarnaev was found hiding after a massive manhunt. AFP
He said the radicalization of his nephew happened "in the streets of Cambridge (Massachusetts)."
More details of the brother's lives were emerging. Tamerlane was married to Katherine Russell, whose family lives in an upper middle-class neighborhood in North Kingstown, Rhode Island.
Her father is licensed as an emergency room physician. The couple had a young child.
The suspected Boston bombers
"She changed her attire completely. When she left, she was a normal American girl and when she returned after the first year, her appearance had changed completely and she wore a headscarf. Soon she had a baby," said Paula Gillette a neighbor of the Russell family in North Kingstown.
A statement on the door of the family's home read: "Our daughter has lost her husband today, the father of her child. We cannot being to comprehend how this horrible tragedy occurred."
Nicole Collier Lynch, sister of slain MIT police officer Sean Collier, during a vigil at the Town Common in Wilmington. Reuters
'Not yet able to speak'
Dzhokhar Tsarnaev was shot in the throat and could not speak because of injuries to his tongue, said a source close to the investigation. It was unclear when he would be able to talk or when he would be charged.
"It's serious ... he's not yet able to speak," Massachusetts Governor Deval Patrick told reporters on Saturday. "We have a million questions and those questions need to be answered."
Boston suspect in serious condition: official
A US justice department official said on Saturday night there would be no further news conference the rest of the day, suggesting Dzhokhar would not be charged before Sunday at the earliest.
The FBI said it did not find any "terrorism activity" when it interviewed Tamerlan in 2011 at the request of a foreign government - identified by a law enforcement source as Russia -after Moscow raised concerns he followed radical Islam. After that, he appears to have dropped off the radar of U.S. authorities.
(Video courtesy: CNN)
President Barack Obama said on Friday after the capture that questions remained from the bombings, including whether the two suspects received any help. Obama has described the bombings an act of terrorism.
On Saturday evening, a vast motorcade of police cars with blue lights flashing escorted the hearse carrying the body of Massachusetts Institute of Technology police officer Sean Collier, 26, along a Boston street. He was shot multiple times in his car on Thursday night as the brothers tried to evade capture.
"It's a good show of solidarity for our brother officer killed by the cowards," said William Gross, a Boston police superintendent. Boston bomb suspect hospitalised under heavy guard
Life in Boston began to return to normal on Saturday as the Red Sox returned to Fenway Park for the first time since the bombings, paying an emotional tribute to the victims and the first responders before their baseball game.
Parents say sons framed
Dzhokhar Tsarnaev was a student at the University of Massachusetts at Dartmouth and is believed to have been on the college campus on Tuesday, Wednesday and Thursday, said a university official, citing witnesses, swipe cards and security cameras.
(Videos courtesy: PBS News)
The family emigrated to the United States about a decade ago. The brothers spent their early years in a small community of Chechens in the central Asian country of Kyrgyzstan, a mainly Muslim nation of 5.5 million.
The family moved in 2001 to Dagestan, a southern Russian province that lies at the heart of a violent Islamist insurgency and where their parents now live. The Brothers Tsarnaev
In separate interviews, the parents of the Tsarnaev brothers said they believed their sons were incapable of carrying out the bombings. Others remembered the brothers as friendly and respectful youths who never stood out or caused alarm.
"Somebody clearly framed them. I don't know who exactly framed them, but they did. They framed them. And they were so cowardly that they shot the boy dead," father Anzor Tsarnaev said in an interview with Reuters in Dagestan's provincial capital, Makhachkala, clasping his head in despair.
How 5-day manhunt unfolded
On Saturday, several Republican lawmakers called on Obama to try Tsarnaev as an "enemy combatant" under terms of war, without entitlement to Miranda rights - usually given by police to suspects before they are interrogated so statements can be admissible in court.
Authorities did not read the teenager the Miranda warning.
(Video courtesy: PBS News)
A Justice Department official said the government was invoking the public safety exception to Miranda to question the suspect extensively about other potential explosive devices or accomplices and to gain critical intelligence. Uncle says older suspect 'used' brother
The Federal Public Defender Office said on Saturday it would represent Tsarnaev once charges were filed.
After combing through a mass of pictures and video from the site in the minutes before the Boston marathon bombing, the FBI publicized pictures of the two men on Thursday and asked the public for help in identifying them.
Just hours later, events began to unfold with the fatal shooting of on the MIT campus and finally the Watertown firefight, during which police say the brothers threw bombs at officers. Tamerlan suffered fatal wounds, while Dzhokhar escaped on foot.