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The FBI was last night hunting a 12-strong terrorist “sleeper cell” linked to the Boston marathon bomb brothers.

Police believe Tamerlan and Dzhokhar Tsarnaev were specially trained to carry out the devastating attack.

More than 1,000 FBI operatives were last night working to track down the cell and arrested a man and two women 60 miles from Boston in the hours before Dzhokhar’s dramatic capture after a bloody shootout on Friday.

A source close to the investigation said: “We have no doubt the brothers were not acting alone. The devices used to detonate the two bombs were highly sophisticated and not the kind of thing people learn from Google.

“They were too advanced. Someone gave the brothers the skills and it is now our job to find out just who they were. Agents think the sleeper cell has up to a dozen members and has been waiting several years for their day to come.”

A specialist team of CIA and FBI interrogators was yesterday flown to a Boston hospital to grill wounded Dzhokhar, 19, about the secret group. The University of Massachusetts student was caught on Friday after hiding out in a boat parked in a garden in locked down Watertown the day after a gun battle with police left his 26-year-old brother and a rookie cop dead.

Dzhokhar is said to have run his brother over as he escaped in a stolen car while Tamerlan lay handcuffed on the ground. They were carrying six bombs with them at the time, three of which ­exploded, as well as a handgun and rifle. The devices were thought to be pipe bombs.

Last night Dzhokhar – badly wounded but alive – lay handcuffed to his hospital bed under armed guard. The other three arrested in the port of New Bedford are also believed to be of college age.

Dzhokhar even went to a college party two days after the bombs wreaked havoc at the finish line. According to fellow students, he “looked relaxed” as he joined in a party at the campus on Wednesday night.

Hours later he was involved in the shootout which saw his brother killed.

Investigators have begun piecing together how the “well-mannered” brothers of Chechen origin were radicalised. Neighbours of the family said older brother Tamerlan had recently become obsessed with Islam. He mysteriously left the US in January last year to spend six months in Russia. Yesterday senior FBI counter-terrorism official Kevin Brock said: “It’s a key thread for investigators.”

It also emerged the Bureau interviewed Tamerlan two years ago, at the request of the Russian government, but could not establish that he had ties to terrorist radicals.

This was despite his worrying Russian-language YouTube page featuring links to extremist Islamic sites and others since taken down by YouTube.

One link showed an hour-long speech by an Islamic teacher called Shaykh Feiz Mohammed, while other videos are labled “Terrorists” and “Islam”.

The radical cleric, with links to extremist British Muslims, encouraged his followers to become martyrs for Islam. He said: “Teach them this: There is nothing more beloved to me than wanting to die as a mujahid.”

Yesterday the brothers’ mother Zubeidat, speaking from her home in Russia, added further intrigue to her sons’ murky past when she claimed the boys had been framed by the FBI over the two bombs last Monday that left three dead and 178 injured.

She claimed the FBI had been keeping watch on her eldest boy for up to five years. She said: “They knew what my son was doing. They knew what sites on the internet he was going to.

“They were telling me that he was really an extremist leader and that they were afraid of him. They told me whatever information he is getting, he gets from these extremist sites. They were controlling him.”

The bombers’ father Anzor wept at news that his youngest son had been captured alive. In a phone interview with a US news channel he told his

son: “Tell police everything. Everything. Just be honest.”

US Government officials have said the brothers were not under surveillance as possible militants. And an FBI statement said the matter was closed because interviews with Tamerlan and family members “did not find any terrorism activity, domestic or foreign”. But now they believe the pair, who emigrated to the United States from Dagestan about a decade ago, were part of a terror cell.

College dropout Tamerlan’s American wife Katherine Russell, 24, and their three-year-old daughter Zahara were yesterday thrown into the spotlight. She was a Christian before they married but converted to Islam. Her parents Warren, a doctor, and Judith were said to be “stunned” by their son-in-law’s involvement in the tragedy.

Judith and Warren issued a joint statement saying: “Our daughter has lost her husband today, the father of her child. In the aftermath of the Patriot’s Day horror, we know we never really knew Tamerlan Tsarnaev. Our hearts are sickened by the horror he has inflicted.”

Katherine, wearing a black hijab, was picked up by FBI agents at their home in Cambridge near Boston on Friday. Dope-smoker Dzhokhar was captured after a Watertown resident called police to say the fugitive was hiding in a boat in his back garden.

David Henneberry had gone into his garden for a cigarette after police lifted restrictions on people leaving their homes, believing the bomber had left the area. He noticed that the cover over his boat had blood on it and a strap had been cut. He went back into the house to get a stepladder and looked inside.

His stepson Robert said: “He stuck his head under the tarp and noticed a pool of blood and something crumpled up in a ball. Instead of being a hero of the moment and yelling at what we now know was the suspect, he did the right thing and called 911.”

Police immediately evacuated the family and surrounded the house, using a megaphone to tell Dzhokhar to come out with his hands up.

When he failed to respond they opened fire at the boat’s hull. Robert said: “They wound up ­shooting a couple of rounds through the boat. He wasn’t going to like that.”

Dzhokhar was wounded by the volley of gunfire and police were able to move in and arrest him. They later released infrared pictures taken from a helicopter showing Dzhokhar hiding in the boat.

Investigators will interrogate the bomber, still seriously ill last night, without reading him his rights – using special “public safety” powers.

The family of eight-year-old bombing victim Martin Richard welcomed the arrest of Tsarnaev. “Our community is once again safe from these men,” the family said in a statement.

Shortly before Dzhokhar’s capture, President Obama spoke by phone to Russian President Vladimir Putin. The White House said Obama “praised the close co-operation the US has received from Russia on counter-terrorism, including in the wake of the Boston attack”.

There were scenes of celebration across Boston as news spread of the capture of the remaining bomber.

Get the latest updates on what happened in Boston.

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- HUNDREDS of extra police called in for today’s London Marathon will remain despite the death and capture of the Boston suspects, Scotland Yard said. Security has been boosted for the 36,000 runners and thousands of spectators.