The teenage suspect in the Boston Marathon bombings is in custody after being found hiding in a boat in a suburban homeowner's backyard.

Police said they exchanged gunfire with Dzhokhar Tsarnaev, 19, after cornering him in Watertown, near Boston.

He had escaped on foot early on Friday, apparently wounded, after a police shootout that claimed the life of his elder brother, an alleged accomplice.

Three people died and more than 170 were hurt in Monday's bombings.

At a Friday night press conference, US President Barack Obama promised to seek answers on what had motivated the bombers and whether they had help.

He spoke just after state police told journalists that the suspect was being treated at a Massachusetts hospital, bleeding and seriously injured with gunshot wounds to the

We may know the names of the suspects in the Boston marathon attacks and also much about their lives, but the question of why they did it remains elusive. Recollections of friends and family, as well as the digital data trail people now leave on social media, means we can get a picture of a young brother who seemed to be a well-adjusted student.

The elder brother comes across less well-adjusted, not quite fitting into American life or making friends. Seven years older, some wonder if he drew his younger sibling into his own world in which he seemed to be exploring radical Islamist ideology.

But was this simply about two brothers, a tight-knit bond with no outside support? An organised conspiracy would worry Americans who hoped that era had passed, but in some ways the era of small self-starting groups is just as worrying since they can be much harder to spot.

The breakthrough came less than an hour after authorities lifted a city-wide order for residents to stay indoors, and reopened the transport system, as the trail appeared to have gone cold.

Authorities captured the suspect following a tip from a resident of Franklin Street, Watertown, who emerged from his home after the lockdown was lifted and noticed blood near the boat.

Upon opening the tarp covering the boat, the resident found a man covered in blood in the stern and called police.

Bomb-squad vans and ambulances surrounded the house, while helicopters buzzed overhead.

Officers tossed flash-bang grenades into the boat to disorient the fugitive.

Police said they exchanged gunfire with the suspect for about an hour before moving in and seizing him.

A crowd near the scene cheered as he was taken into custody.

The property was apparently not searched earlier on Friday as police went door-to-door in Watertown.

Boston Police Department tweeted: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."

Thousands of Swat team officers had earlier scoured the streets all day in a manhunt that virtually shut down the city.
Russian warning?

Massachusetts officials had closed all mass transit and warned nearly one million people in Boston and some of its suburbs not to leave their homes.

Dzhokhar Tsarnaev, a college student, had fled on foot following a gun battle that left 200 spent rounds and a car chase to Watertown in which he and his brother, 26-year-old Tamerlan Tsarnaev, hurled explosives at police, authorities said.

The pair also shot and killed a Massachusetts Institute of Technology policeman late on Thursday before severely wounding another officer, authorities said.

The elder brother died of bullet wounds and injuries from explosives strapped to his body, a hospital doctor said.

Their bloody confrontation ensued hours after the FBI released images of the marathon-bombing suspects.

Law enforcement officials and family members have identified the Tsarnaev brothers as ethnic Chechens who had been living in America for about a decade.

The FBI interviewed Tamerlan Tsarnaev in 2011 after a request from a foreign government, US law enforcements officials have confirmed. But agents closed the case after finding no cause for concern.

In an interview on Russian television, the mother of the two suspects said the FBI had been in contact with her son for several years.

It is not known which country made the request, but the BBC's Paul Adams in Washington says it is likely to have come from the Russians.
'Losers'

Our correspondent says now that the manhunt in Boston is over, the extent of the FBI's prior knowledge of Tamerlan Tsarnaev's activities is likely to be examined.

Monday's twin blasts killed Martin Richard, aged eight, Krystle Campbell, 29, and Lu Lingzi, 23, a postgraduate student from China.

Earlier on Friday, the father of Dzhokhar Tsarnaev said his son was a second-year medical student in the US and was hoping to be a brain surgeon.

Anzor Tsarnaev told the BBC he believed the secret services had framed his sons.

"It was a terrorist attack carefully organised by secret services - I don't know which ones. My son used to go to a mosque, so they once paid us a visit to ask why he is doing that.

"Yes, there was such an episode. So they put all the blame on him and shot him. That's it."

But Ruslan Tsarni, an uncle of the suspects who lives in Maryland, said he was "ashamed" at his nephews' alleged role in the bombings.

Asked what the bombers' motives may have been, he replied: "Being losers, hating everyone around them."

Also on Friday, investigators removed a computer and other evidence from the New Jersey home of the Tsarnaev brothers' sister, police said.

Ailina Tsarnaeva, who lives in the town of West New York, is said to have told FBI agents she had had no contact with her brothers for some time.

BBC