With the surviving suspect in the Boston Marathon bombings lying seriously wounded in a hospital, investigators worked on Saturday to find a motive and whether the ethnic Chechen brothers accused in the attack acted alone.
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, was killed on Thursday in a shootout with police.
Tsarnaev was being treated at Beth Israel Deaconess Medical Center in Boston, said spokeswoman Kelly Lawman, who said the FBI would provide updates on his condition. It was not clear when he would be charged.
Tsarnaev had been hiding in a boat parked in the backyard of a house in the suburb of Watertown, police said. A resident called police after spotting blood on the boat. Police said he was bleeding and in serious condition when admitted to hospital.
U.S. President Barack Obama said after the capture that questions remained from the bombings, including whether the two suspects received any help.
Early indications are that the suspects acted alone, the 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 cell or this little group, I think we got our guys."
Monday's bombing at the world-famous Boston Marathon was described by Obama as an act of terrorism and was the worst such attack on U.S. soil since September 11, 2001.
The brothers are suspected of setting off bombs made in pressure cookers and packed with ball bearings and nails at the crowded finish line, killing three people and injuring 176.
The family of 8-year-old bombing victim Martin Richard welcomed the arrest of Tsarnaev. "Our community is once again safe from these two men," the family said in a statement.
The brothers had not been under surveillance as possible militants, U.S. government officials said. But the FBI said on Friday that it interviewed Tamerlan in 2011 at the request of a foreign government, which it did not identify.
The request stated that it was based on information that he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010 as he prepared to leave the United States for travel to the country's region to join unspecified underground groups," the FBI statement said.
The matter was closed when the FBI "did not find any terrorism activity, domestic or foreign."
The FBI statement was the first evidence that the family had come to security officials' attention after they 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.
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.
The mother, Zubeidat Tsarnaeva, told Russia Today state television: "It's impossible, impossible, for both of them to do such things, so I am really, really, really telling that this is a setup."
The Russian-installed leader of Chechnya criticized police in Boston for killing an ethnic Chechen and blamed the violence on his upbringing in the United States.
The Islamic Society of Boston Cultural Center, the biggest mosque in the area, said in a statement that it was shutting its doors until further notice.
The bombings prompted contact between the United States and Russia on terrorism and the Kremlin said on Saturday that the presidents of both had agreed by telephone to increase cooperation on counter-terrorism.
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 fast with the fatal shooting of a police officer on the campus of the Massachusetts Institute of Technology and finally the Watertown firefight, during which police say the brothers threw bombs at officers. Tamerlan suffered fatal wounds, while Dzhokhar escaped on foot.
At the same time that police were pursuing Dzhokhar on Friday night, police in New Bedford, Massachusetts, 96 kilometers (60 miles) south of Boston said three other people had been taken into custody for questioning about Monday's bombings. They were later released, police said.
The hunt for Tsarnaev emptied Boston's streets as the city went into lockdown for most of Friday. Public transportation was suspended and air space restricted. Famous universities, including Harvard and MIT, closed after police told residents to remain at home.
Authorities did not immediately read the teenager a warning usually given by police to criminal suspects in custody before they are interrogated so statements can be admissible in court.
A Justice Department official said the government is invoking the public safety exception to the warning, known as Miranda rights, in order to question the suspect extensively about other potential explosive devices or accomplices and to gain critical intelligence.
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