Police guarded Boston bombing suspect Dzhokhar Tsarnaev in hospital on Saturday as detectives searched for clues to how the Chechen teenager and his brother could have turned to terrorism.
Tsarnaev was in "serious condition" in a hospital where some victims of Monday's marathon bombing are also being treated, after he was found hiding in a boat at the end of a manhunt that paralyzed Boston.
The 19-year-old was wounded in an earlier shootout in which his 26-year-old brother Tamerlan Tsarnaev - who has been linked to Islamic extremism through social media - was killed.
Prosecutors were at the Beth Israel Deaconess Hospital in Cambridge, just outside Boston, developing formal charges, officials said.
US authorities now face tough decisions over how to handle the investigation and any trial of Tsarnaev for the marathon bombs, which left three dead and about 180 injured, including several victims with severed limbs.
A policeman was killed and another critically wounded during the hunt for the brothers, who are accused of the worst attack on the United States since the September 11, 2001 atrocities.
Republican senators John McCain and Lindsay Graham have called for the teenager to be declared an "enemy combatant," which would give him the same status as Guantanamo "War on Terror" detainees.
Russia's President Vladimir Putin and US President Barack Obama discussed the Boston attacks on Saturday because of the brothers' origins in Russia's volatile North Caucasus region.
The two leaders agreed to step up security and counter-terrorism cooperation in the wake of the attacks, according to the White House and the Kremlin.
The younger Tsarnaev was caught after a resident in Watertown in the Boston suburbs saw blood on a boat he kept in his backyard, lifted the cover and saw the bloodied teenager curled up inside, police said.
A small army of police surrounded the University of Massachusetts student for a showdown that lasted nearly two hours. Tsarnaev gave up to Federal Bureau of Investigation (FBI) agents after a final gunbattle.
"The terror is over. And justice has won," the Boston police department said on Twitter, as the city's residents poured into the streets to celebrate, chanting: "USA! USA!" Some climbed onto car roofs to applaud police.
Boston became a virtual city under siege Friday as thousands of police hunted for the brothers after images of them near the bomb scene were released.
Authorities told hundreds of thousands of people to stay in their homes, halted public transport and closed schools and many businesses.
In a rampage earlier Friday, a police officer was killed in what Boston police commissioner Ed Davis called a "vicious assassination." The suspects then carjacked a Mercedes and hurled explosives at police chasing them.
Tamerlan Tsarnaev died after a shootout in which he was wounded by police fire and by explosives he had strapped to his body. His brother was also hit but escaped.
There is now intense scrutiny into how the brothers, ethnic Chechens who moved to the United States around a decade ago, may have turned to Islamist militancy.
The FBI acknowledged on Friday that an unnamed foreign government had asked about Tamerlan Tsarnaev in 2011. The FBI interviewed the man but said in a statement they had found no "derogatory" information.
In his weekly radio address Saturday, Obama said police had been "heroic" and that the response had shown that "Americans refuse to be terrorized."
But in comments immediately after the capture, Obama said: "Obviously, there are still many unanswered questions.
"Among them, why did young men who grew up and studied here as part of our communities and our country resort to such violence? How did they plan and carry out these attacks? And did they receive help?"
Some reports say Tamerlan Tsarnaev had become a fervent Muslim who spent several months in Russia last year and had influenced his brother.
While Tamerlan briefly studied engineering at a community college, his younger brother was a scholarship-winning student and all-star wrestler who attended the University of Massachusetts, Dartmouth.
Many of the younger man's teachers and friends have expressed shock at his involvement in the bombing.
The men's social media pages appeared to express sympathy with the struggle of Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
The suspects' father, Anzor Tsarnaev, told CNN from Dagestan that he planned to return to the United States to see his son.
He had earlier told Russia's Interfax news agency his sons had been "set up by the secret services because they are practicing Muslims."