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.
President Barack Obama said there are many unanswered questions about the bombing, including whether the Tsarnaev brothers ethnic Chechens from southern Russia who had been in the U.S. for about a decade and lived in the Boston area had help from others. The president urged people not to rush judgment about their motivations.
U.S. officials said a special interrogation team for high-value suspects would question the Massachusetts college student without reading him his Miranda rights, invoking a rare public-safety exception that exists in cases of immediate danger.
The American Civil Liberties Union expressed concern about that possibility. Executive Director Anthony Romero said the exception applies only when there is a continued threat to public safety and is "not an open-ended exception" to the Miranda rule, which guarantees the right to remain silent and the right to an attorney.
The federal public defender's office in Massachusetts said it has agreed to represent Tsarnaev once he is charged. Miriam Conrad, public defender for Massachusetts, said he should have a lawyer appointed as soon as possible because there are "serious issues regarding possible interrogation."
Massachusetts Gov. Deval Patrick said Saturday afternoon that Tsarnaev was in serious but stable condition and was probably unable to communicate.
"I, and I think all of the law enforcement officials, are hoping for a host of reasons the suspect survives," Patrick said after a pregame ceremony at Fenway Park to honor the victims and survivors of the attack. "We have a million questions, and those questions need to be answered."
The all-day manhunt Friday brought the Boston area to a near standstill and put people on edge across the metropolitan area.
The break came around nightfall when a homeowner in Watertown saw blood on his boat, pulled back the tarp and saw a bloody Dzhokhar Tsarnaev hiding inside, police said. After an exchange of gunfire, he was seized and taken away in an ambulance.
Raucous celebrations erupted in and around Boston, with chants of "USA! USA!" Residents flooded the streets in relief four days after the twin explosions ripped through the marathon crowd at the finish line, killing three people and wounding more than 180.
Michael Spellman said he bought tickets to Saturday's Red Sox game at Fenway Park to help send a message to the bombers.
"They're not going to stop us from doing things we love to do," he said, sitting a few rows behind home plate. "We're not going to live in fear."
During the long night of violence leading up to the capture, the Tsarnaev brothers killed an MIT police officer, severely wounded another lawman and took part in a furious gun battle and car chase in which they hurled explosives at police from a large homemade arsenal, authorities said.
Chechnya, where the Tsarnaev family has roots, has been the scene of two wars between Russian forces and separatists since 1994. That spawned an Islamic insurgency that has carried out deadly bombings in Russia and the region, although not in the West.
Investigators have not offered a motive for the Boston attack. But in interviews with officials and those who knew the Tsarnaevs, a picture has emerged of the older one as someone embittered toward the U.S., increasingly vehement in his Muslim faith and possessed of a strong influence over his younger brother.
The Russian FSB intelligence security service told the FBI in 2011 about information that Tamerlan Tsarnaev was a follower of radical Islam, two law enforcement officials said Saturday.
According to an FBI news release, a foreign government said that Tamerlan Tsarnaev appeared to be strong believer and that he had changed drastically since 2010 as he prepared to leave the U.S. for travel to the Russian region to join unspecified underground groups.
The FBI did not name the foreign government, but the two officials said it was Russia. The officials spoke on condition of anonymity because they were not authorized to talk about the matter publicly.
The FBI said that in response, it interviewed Tamerlan Tsarnaev and relatives, and did not find any domestic or foreign terrorism activity. The bureau said it looked into such things as his telephone and online activity, his travels and his associations with others.
An uncle of the Tsarnaev brothers said he had a falling-out with Tamerlan over the man's increased commitment to Islam.
Ruslan Tsarni of Montgomery Village, Md., said Tamerlan told him in a 2009 phone conversation that he had chosen "God's business" over work or school. Tsarni said he then contacted a family friend who told him Tsarnaev had been influenced by a recent convert to Islam.
Tsarni said his relationship with his nephew ended after that call.
As for Dzhokhar Tsarnaev, "he's been absolutely wasted by his older brother. I mean, he used him. He used him for whatever he's done," Tsarni said.
Albrecht Ammon, a downstairs-apartment neighbor of Tamerlan Tsarnaev in Cambridge, said in an interview that the older brother had strong political views about the United States. Ammon quoted Tsarnaev as saying that the U.S. uses the Bible as "an excuse for invading other countries."
Tamerlan Tsarnaev studied accounting as a part-time student at Bunker Hill Community College in Boston for three semesters from 2006 to 2008, the school said. He was married with a young daughter.
Dzhokhar Tsarnaev was a student at the University of Massachusetts Dartmouth. Students said he was on campus this week after the Boston Marathon bombing.
As of Saturday, more than 50 victims of the bombing remained hospitalized, three in critical condition.
Doctor: Dead bomb suspect had wounds 'head to toe'
A doctor involved in treating the Boston Marathon bombing suspect who died in a gunbattle with police says he had injuries head to toe and all limbs intact when he arrived at the hospital.
Dr. David Schoenfeld says 26-year-old Tamerlan Tsarnaev was unconscious and had so many penetrating wounds that it isn't clear which ones killed him. The doctor says a medical examiner will have to determine the cause of death.
Schoenfeld says the suspect was in cardiac arrest and lost a pulse as soon as he arrived at Beth Israel Deaconess Medical Center following a shootout with police in the Boston suburb of Watertown early Friday. Doctors tried numerous treatments for about 15 minutes before pronouncing him dead.
The doctor said "we did everything we could" to try to save his life.
A look at Boston Marathon bombing, investigation
Two explosions near the finish line of the Boston Marathon on April 15 killed three people and wounded scores. Authorities launched a massive manhunt for two suspects seen on surveillance video. A look at the basics of the case:
Bombing suspect Dzhokhar Tsarnaev was hospitalized Saturday in serious but stable condition after having been found wounded and bloody the night before hiding in a boat parked behind a home in Watertown, a Boston suburb authorities had shut down to conduct house-by-house searches.
U.S. officials said a special interrogation team for high-value suspects would question Tsarnaev without reading him his Miranda rights, invoking a rare public-safety exception triggered by the need to protect police and the public from immediate danger. The American Civil Liberties Union said it was concerned about that. It said the exception applies only when there's a continued threat to public safety and is "not an open-ended exception" to the Miranda rule, which guarantees a suspect the right to remain silent and the right to an attorney.
Federal public defenders in Massachusetts have agreed to represent Tsarnaev and said they expect to do so after he is charged. There was no immediate word on when or with what crimes Tsarnaev might be charged, but 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 upon conviction.
The FBI said a foreign government told it in early 2011 about information that Dzhokhar Tsarnaev's older brother, Tamerlan Tsarnaev, was a follower of radical Islam.
According to the FBI, the foreign government said Tamerlan Tsarnaev had changed drastically since 2010 as he prepared to leave the United States to join unspecified underground groups. The FBI says it interviewed him and relatives and didn't find any terrorism activity.
How the situation unfolded
Surveillance tape late Thursday showed Dzhokhar Tsarnaev, Suspect No. 2, at a gas station. The two suspects fatally shot a university police officer, authorities say. They carjacked a man, released him and got involved in a chase with police that resulted in explosives being thrown from their car and an exchange of gunfire, authorities say. Tamerlan Tsarnaev, Suspect No. 1, was wounded and died later at a hospital.
After the death of one suspect and the capture of the other, people in Boston and its western suburbs thanked police, cheered, applauded and set off fireworks. Later, they gathered in silence near the site of the bombings to remember the victims. Some cried. Some wrapped themselves in American flags.
"Now I feel a little safer," Boston resident Beth Lloyd-Jones said.
Law enforcement officials and family members have identified the brothers as ethnic Chechens who had lived in Dagestan, in southern Russia. The brothers had been in the United States for about a decade and lived near Boston, an uncle said. Dzhokhar Tsarnaev, whose father has called him a "true angel," is a 19-year-old student at the University of Massachusetts at Dartmouth. Tamerlan Tsarnaev was a 26-year-old boxer. Their uncle Ruslan Tsarni called them "losers."
The marathon explosions
Two bombs exploded about 10 seconds and 100 yards apart in Boston's Copley Square, near the finish line of the marathon. An 8-year-old boy, a 29-year-old woman and a 23-year-old graduate student from China were killed, and more than 180 people were wounded. The explosions occurred about four hours into the race and two hours after the winners had crossed the finish line, but thousands of runners were still on the course.
Authorities have said they believe the bombs used were fashioned out of ordinary kitchen pressure cookers packed with explosives, nails, ball bearings and metal shards. They say the bombs were hidden in backpacks and left on the ground.