Police captured the surviving Boston Marathon bombing suspect, found bloodied in a backyard boat after a wild car chase and gun battle that left his older brother dead and the Boston area sealed in an extraordinary dragnet.
The capture of Dzhokhar Tsarnaev - taken alive, though seriously wounded- lifted days of anxiety for Boston and Americans everywhere, but little was known about the motivation of the ethnic Chechen brothers.
US President Barack Obama vowed investigators would solve that mystery. "The families of those killed so senselessly deserve answers," said Obama, who branded the suspects "terrorists."
During a long night of violence on Thursday and into Friday, the brothers killed a Massachusetts Institute of Technology police officer, severely wounded another lawman and hurled explosives at police in a desperate getaway attempt, authorities said.
Late Friday, less than an hour after authorities said the search for Dzhokhar had proved fruitless, they tracked down the 19-year-old university student holed up in the boat.
He was weakened by a gunshot wound after fleeing on foot from the overnight shootout with police that left 200 spent rounds behind.
Tsarnaev was hospitalized in serious condition, unable to be questioned about his motives.
Boston police announced via Twitter that Tsarnaev was in custody. They later wrote: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."
Tamerlan Tsarnaev, 26, died in the shootout early in the day. At one point, he was run over by his younger brother in a car as he lay wounded, according to investigators.
The bloody endgame came four days after the bombing and just a day after the FBI released surveillance-camera images of two young men suspected of planting the pressure-cooker explosives that ripped through the crowd at the marathon finish line, killing three people and wounding more than 180.
The two men were identified by authorities and relatives as ethnic Chechens from southern Russia who had been in the US for about a decade and were believed to be living in Cambridge, Massachusetts. But investigators gave no details on the motive for the bombing.
This photo shows (third from left) Tamerlan Tsarnaev, the first suspect and (second from left) Dzhokhar A. Tsarnaev, the second suspect, in the Boston Marathon bombings by law enforcement. (AP)
Obama said the capture closed "an important chapter in this tragedy," but he said there are many unanswered questions about the Boston bombings, including whether the two men had help from others. He urged people not to rush judgment about their motivations.
"When a tragedy like this happens, with public safety at risk and the stakes so high, it's important that we do this right," he said. "That's why we take care not to rush to judgment - not about the motivations of these individuals, certainly not about entire groups of people."
The breakthrough came when a man in Watertown saw blood on a boat parked in a yard and pulled back the tarp to see a man covered in blood, authorities said.
Dzhokhar Tsarnaev, a suspect in the Boston Marathon bombing who was captured after he was found hiding in a boat in a Boston suburb. (AFP/CBS News)
The resident called authorities and when police arrived, they tried to talk the suspect into getting out of the boat, said Boston Police Commissioner Ed Davis.
"He was not communicative," Davis said.
Instead, he said, there was an exchange of gunfire - the final volley of one of the biggest manhunts in American history.
Watertown residents who had been told in the morning to stay inside behind locked doors poured out of their homes and lined the streets to cheer police vehicles as they rolled away from the scene.
Celebratory bells rang from a church tower. Teenagers waved American flags. Drivers honked. Every time an emergency vehicle went by, people cheered loudly.
A crowd gathers at Boston Common after the final suspect in the Boston Marathon bombing was arrested in Boston. (AP)
Police said three other people were taken into custody for questioning at an off-campus housing complex at the University of the Massachusetts at Dartmouth where the younger man may have lived.
"Tonight, our family applauds the entire law enforcement community for a job well done, and trust that our justice system will now do its job," said the family of 8-year-old Martin Richard, who died in the bombing.
Also killed in the attack was a Chinese student.
Chechnya has been the scene of two wars between Russian forces and separatists since 1994, in which tens of thousands were killed in heavy Russian bombing.
That spawned an Islamic insurgency that has carried out deadly bombings in Russia and the region, although not in the West.
The older brother had strong political views about the United States, said Albrecht Ammon, 18, a downstairs-apartment neighbor in Cambridge. Ammon quoted Tsarnaev as saying that the US uses the Bible as "an excuse for invading other countries."
The FBI interviewed the older brother at the request of a foreign government in 2011, and nothing derogatory was found, according to a federal law enforcement official who was not authorized to discuss the case publicly and spoke on condition of anonymity.
A gathering of people including the media gather around a police officer as he leaves the scene after the arrest of a suspect of the Boston Marathon bombings in Watertown. (AFP)
The official did not identify the foreign country or say why it made the request.
The FBI was swamped with tips after the release of the surveillance-camera photos - 300,000 per minute - but what role those played in the capture was unclear. State Police spokesman Dave Procopio said police realized they were dealing with the bombing suspects based on what the two men told a carjacking victim during their long night of crime.
The search for the younger brother all but paralyzed the Boston area for much of the day. Officials shut down all mass transit, including Amtrak trains to New York, advised businesses not to open, and warned close to 1 million people in the entire city and some of its suburbs to stay inside and unlock their doors only for uniformed police.
Around midday, the suspects' uncle Ruslan Tsarni, of Maryland, pleaded on television: "Dzhokhar, if you are alive, turn yourself in and ask for forgiveness."
The brothers had built an arsenal of pipe bombs, grenades and improvised explosive devices and used some of the weapons in trying to make their getaway, said Rep. Dutch Ruppersberger, a member of the House of Representatives Intelligence Committee.
Tamerlan Tsarnaev had studied accounting as a part-time student at Bunker Hill Community College in Boston for three semesters from 2006 to 2008, the school said.
Dzhokhar Tsarnaev was registered as a student at the University of Massachusetts Dartmouth. Students said he was on campus this week after the Boston Marathon bombing. The campus closed down Friday along with colleges around the Boston area.
This wanted poster was released by the FBI showing Dzhokhar Tsarnaev at the Boston Marathon. (AP/CBS News)
The men's father, Anzor Tsarnaev, said in a telephone interview with AP from the Russian city of Makhachkala that his younger son, Dzhokhar, is "a true angel." He said his son was studying medicine.
"He is such an intelligent boy," the father said. "We expected him to come on holidays here."
The city of Cambridge announced two years ago that it had awarded a $2,500 scholarship to him. He was then a senior at Cambridge Rindge & Latin School, a highly regarded public school whose alumni include Matt Damon and Ben Affleck.
Tsarni, the men's uncle, said the brothers traveled here together from Russia. He called his nephews "losers" and said they had struggled to settle in the US and ended up "thereby just hating everyone."
Police officers guard the entrance to Franklin street where there is an active crime scene search for the suspect in the Boston Marathon bombings in Watertown. (AP)
Shortly before Dzhokhar Tsarnaev's capture, the White House said Obama had spoken by phone with Russian President Vladimir Putin about the investigation.
The White House said in a statement that Obama "praised the close cooperation that the United States has received from Russia on counter-terrorism, including in the wake of the Boston attack."
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