The surviving suspect in the Boston Marathon bombing is under armed guard tonight, as special interrogators prepare to question him if and when his condition improves.

Dzhokhar Tsarnaev, 19, is currently not fit to be questioned, according to officials. He was caught yesterday after a tense hunt in a Massachusetts suburb, four days after the attack which killed three and wounded more than 150.

The team planned to question Tsarnaev without reading him the Miranda rights - the statement usually read by police to suspects stating their right to remain silent and to a lawyer. Authorities plan to invoke a rare public safety exception triggered by the need to protect police and the public from immediate danger.

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The FBI says "police officers confronting situations that create a danger to themselves or others may ask questions designed to neutralize the threat without first providing a warning of rights."

But Anthony Romero, Executive Director of the American Civil Liberties Union, said the exception applies only when there's a continued threat to public safety and is "not an open-ended exception."

The manhunt ended after nightfall on Friday, almost 24 hours since the police first got on the trail of the two suspects, Dzhokhar and his 26-year-old brother Tamerlan, immigrants of Chechen origin who lived next door in Cambridge.

Dzhokhar Tsarnaev was taken away on a stretcher and was admitted to hospital in a serious condition with unspecified injuries, police said. He remains seriously ill and unable to be questioned.

News of his capture led to jubilant celebrations at the scene and across the city.

In a televised address from the White House, Barack Obama said Dzhokhar's capture "closed an important chapter in this tragedy".

But he acknowledged that many unanswered questions remain about the motivations of the two men accused of perpetrating the attacks that unnerved the nation.

"The families of those killed so senselessly deserve answers," said Mr Obama, branding the suspects "terrorists".

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On Thursday evening, they killed a campus policeman on the grounds of the Massachusetts of Institute Technology. Sean Collier, 26, was fatally shot when he responded to a report of a disturbance on the university grounds, officials said.

The Tsarnaev brothers then hijacked a car and briefly captured a hostage before being pursued down Memorial Drive, across the Charles river from Boston, by the police. At some point during the high-speed chase, which saw the brothers return fire and throw explosives at the police, they stopped and engaged in a shootout with officers which led to Tamerlan’s death. A transit police officer was also injured in the encounter and was in surgery on Friday.

Dzhokhar then drove off before abandoning his car and disappearing into Watertown, leading to a massive manhunt involving some 9,000 officers from state and federal law enforcement agencies, including the Federal Bureau of Investigation and the US Department of Homeland Security.

The wider Boston metropolitan area was paralysed by the search, as officials shut down the public transportation system and asked residents to remain indoors.

In Watertown, which sits just west of Cambridge, officers cordoned off some twenty blocks in the early hours of Friday morning, going door by door in their hunt for the suspect.

But at dusk it seemed he had escaped, with Col Timothy Alben of the Massachusetts state police saying that authorities did not know where he went following Thursday’s chase. “He fled on foot. I don’t know where he went after that specifically,” he conceded as the Boston area lockdown was lifted shortly after 6pm on Friday.

However, less than an hour after Col Alben’s announcement, gunshots were heard in the Franklin Street area of Watertown. A resident who had been holed up inside his house through the day had come out to find traces of blood near or on the boat in his back yard. Police said that when the resident lifted the tarp, he saw a man covered with blood.

A call to the emergency services saw the swift arrival of droves of law enforcement personnel. With military helicopters hovering overhead, authorities asked Watertown residents to go back indoors. A brief stand-off ensued, with officers exchanging fire with the 19-year-old before apprehending him. Boston Police Commissioner Ed Davis said Dzhokhar Tsarnaev had serious injuries when he was caught and was rushed to hospital. The wounds are thought to have been sustained during Thursday night’s shootout.

“After a vicious attack on their city, the people of Boston responded with resolve and determination,” President Obama said, speaking from the White House soon after the arrest had been confirmed. Of the thousands of law enforcement personnel involved, he said: “Tonight because of their determined efforts we’ve closed an important chapter in this tragedy.”

Three people died and more than 170 were injured when authorities say the Tsarnaev brothers planted two home-made bombs near the finish line of the Boston Marathon in the heart of the city. Eight-year old Martin Richard, 29-year old Kyrstle Campbell and 23-year old Lu Lingzi, a Boston University graduate student from China, were killed when the bombs went off in quick succession on Monday afternoon.

No motive has yet been identified for the brothers’ actions.