Major questions about US security and intelligence services were raised last night after it was revealed that the elder of the two apparent Boston bombers had been investigated as a possible Islamist terrorist two years ago, but was not deemed worthy of further attention.

The FBI has said that in 2011, at the request of a foreign government it did not name, it interviewed Tamerlan Tsarnaev, who was killed on Thursday night after he and his brother hijacked a car in an attempt to break out of the military and police cordon around parts of Boston. A bureau statement said the request "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". It is believed that the request was made by Russia.

The statement added that its interviews with Tsarnaev and his family, along with checks of travel records, internet activity and personal associations, "did not find any terrorism activity" at the time. Representative Michael McCaul, Texas Republican and chairman of the House Homeland Security Committee, said: "It's new information to me and it's very disturbing that he's on the FBI radar."

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Last night, Zubeidat Tsarnaeva, mother of the brothers, told Russia Today that the family was subject to "constant surveillance... over the years". She was quoted as saying: "They used to come to our home, they used to talk to me... they were telling me that he [Tamerlan] was really an extremist leader and that they were afraid of him."

National security and law enforcement authorities said they had not turned up any evidence that the Tsarnaevs had contacts with al-Qa'ida or other militants overseas. They were now leaning toward the theory that the bombings, in which three people died, were motivated by Islamist extremism, although that remained unproven.

Tamerlan, 26, and his brother, Dzhokhar, 19, may be still called "suspects", but US authorities are certain it was they who planted the bombs on Monday. Dzhokhar is in hospital after being captured on Friday evening as he hid in a tarpaulin-covered boat stored in a suburban backyard.

Massachusetts Governor Deval Patrick said last night Dzhokhar was in a “serious but stable” condition, although unable to speak. He gave an impromptu interview outside Fenway Park, the home of the Boston Red Sox baseball team, telling reporters: “I, and I think all of the law enforcement professionals, are hoping for a host of reasons that the suspect survives because we have a million questions and those questions need to be answered.”

Tamerlan had, according to several relatives, become a far more devout Muslim in recent years, praying five times a day and criticising the US from a religious point of view. The YouTube channel "Tamerlan Tsarnaev" has nearly 700 subscribers, and shows Islamist videos. One playlist is called "Terrorists" and contained two videos, now removed. The other, "Islam", features seven videos, including "The Emergence of Prophecy: The Black Flags From Khorasan", which has been linked to jihadist ideology.

Albrecht Ammon, 18, who lived near the two brothers, said he recently saw Tamerlan and they argued about religion and US foreign policy. He quoted Tsarnaev as saying that many US wars are based on the Bible, which is used as "an excuse for invading other countries". During the argument, Ammon said, Tsarnaev told him he had nothing against the American people, but he had something against the American government. "The Bible was a cheap copy of the Koran," Ammon quoted Tsarnaev as saying.

Tamerlan was married to Katherine Russell, and the couple had a daughter. Judith Russell, Katherine's mother, said: "In the aftermath of the Patriots' Day horror, we know that we never really knew Tamerlan Tsarnaev." A neighbour of Judith Russell, Paula Gillette, said Katherine had recently dressed in less revealing clothing with head coverings.

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The brothers' uncle, Ruslan Tsarni, who lives in Maryland, said he had not spoken to Dzhokhar and Tamerlan since 2009, or seen them since 2005. He said Tamerlan had become a devout Muslim some years ago. "When I was speaking to the older one, he started all this religious talk, 'Insh'allah' and all that, and I asked him, 'Where is all that coming from?"' Mr Tsarni told reporters.

But he seemed to think the explanation for the bombings lay less with religion and more with personal resentment. Mr Tsarni said his nephews had struggled to settle in the US and ended up "thereby just hating everyone". Asked what he thought provoked the bombings, Mr Tsarni said: "Being losers, hatred to those who were able to settle themselves... Anything else, anything else to do with religion, with Islam, it's a fraud, it's a fake."

Another source of possible ill-feeling was suggested by the men's father. Tamerlan was, according to law enforcement records, arrested in 2009 for assault and battery on a girlfriend. Charges were dismissed, but his father said the case thwarted Tamerlan's hopes for US citizenship.

The Tsarnaev brothers were ethnic Chechens who were born in Kyrgyzstan and went to school in Dagestan, before Dzhokhar emigrated as a refugee to the US with his parents in 2002. At the time, his elder brother and two sisters were in Kazakhstan but later joined the family in the US. The father set up as a car mechanic, and the two boys went to school. Dzhokhar, at least, attended the Cambridge Rindge and Latin School, a prestigious state school. The father then went back to Dagestan with the mother, who has travelled back and forth to the US.

The boys' paths diverged somewhat – at least for a while. Tamerlan dropped out after studying accounting at Bunker Hill Community College for just three terms, in autumn 2006, the following spring, and then the autumn of 2008. He was quoted in a Boston University student magazine in 2010 saying: "I don't have a single American friend. I don't understand them." He identified himself then as a Muslim and said he did not drink or smoke.

Dzhokhar, meanwhile, was registered as a student at the University of Massachusetts Dartmouth. More than anything, he wanted to be popular, according to those who knew him. Friends and acquaintances say he tried hard to get along with everyone. He used the word "dude". He was cheery, nervous and socially awkward, but not in a way that made people uncomfortable. And he didn't talk much about politics. Nate Mann, 20, who was in the class above him at school, said: "Seriously, he was so, so normal, no accent, an all-American kid in every measurable sense of the word."

He was not, apparently, a successful student. The New York Times reported that a university transcript revealed that he was failing many of his college classes. In two semesters in 2012 and 2013, he got seven failing grades, including Fs in Principles of Modern Chemistry, Intro to American Politics, and Chemistry and the Environment. He had, however, taken a keen interest in Chechen history at school. Dr Brian Glyn Williams, who tutored Dzhokhar in the subject, said: "He was learning his Chechen identity, identifying with the diaspora and identifying with his homeland … He wanted to learn more about Chechnya, who the fighters were, who the commanders were."

In separate interviews, the brothers' parents said they believed their sons incapable of carrying out the bombings. Anzor Tsarnaev said: "Somebody clearly framed them. I don't know who... but they did. And they were so cowardly that they shot the boy dead." Their mother, Zubeidat Tsarnaeva, told CNN: "It's impossible, impossible, for both of them to do such things, so I am really, really, really telling that this is a set-up."