Federal prosecutors are trying to piece together the complex web of influences that transformed a young man with no confirmed militant training or links, apparently acting alone with only the assistance of his younger brother, into a brutal bomber prepared to kill and maim in pursuit of a cause that remained largely unarticulated.

Tamerlan Tsarnaev has become the focal point of a global FBI investigation into whether any organised group or wider conspiracy lay behind last week's Boston Marathon bombings. The 26-year-old, who has been identified through fingerprinting as the man killed in the shootout with police in the Watertown suburb of Boston, is widely assumed to have been the mastermind of the marathon outrage, with his younger brother Dzhokhar Tsarnaev allegedly playing the role of junior partner.

Yet so far the hunt for clues as to the motivation of the Tsarnaev brothers has failed to throw up concrete evidence that they were inspired to militancy by any particular extremist cleric or politician. Nor is there any known link to any nationalist or Islamist group in the Caucasus region that they regarded as their homeland, a link which would suggest they were recruited as foot soldiers and given operational instructions to strike the Boston Marathon.

In the absence of any firm connections to inspirational leaders or terrorist groups, federal investigators and counter-terrorism experts are increasingly of the view that the brothers were acting alone. The surmise is that the elder Tsarnaev largely provided his own motivation and training through the internet.

The idea that the marathon bombings may have been carried out by two men acting as "lone wolves" underlines the daunting task facing counter-terrorism authorities in the US and across the western world. Individual would-be bombers are far more difficult to detect than those operating on behalf of organised groups.

"If there is a link to a terrorist organisation the probability of them being detected is much higher. The same applies for homegrown terrorism – the more people are involved the more likely they will be detected," said Marc Sageman, a former CIA operations officer who acts as a consultant on political violence to several US government branches.

'Down the rabbit hole'

In this case though, the two brothers appeared, from current understanding of their actions, to have kept their plans entirely to themselves. Tamerlan Tsarnaev was married to Katherine Russell, 24, a convert to Islam who has claimed through her lawyer that she learned her husband and brother-in-law were the accused Boston Marathon bombers from TV.

Yuri Zhukov, a national security fellow at Harvard's Weatherhead Center for International Affairs, said the radicalisation process of Tamerlan Tsarnaev "didn't fit the profile that security people in this country would detect. He was apparently not connected to Emirate of the Caucasus or Chechen groups in any formal way, as far as we can tell he didn't receive terrorist training and he didn't attend a mosque on a regular basis."

Zhukov has analysed some 30,000 violent attacks waged in the Caucusus region and found that in most cases there is substantial chatter between instigators followed by a claim of responsibility for the incident – none of which has been uncovered in the Boston Marathon bombings. One of the most prominent separatists groups in the region, the Caucasus Emirate in Dagestan, where the brother's father Anzor Tsarnaev lives, has publicly denied any connection to the Boston attacks.

Counter-terrorism experts are coming increasingly to the view that Tamerlan Tsarnaev radicalised himself, developing his own ideological passions and training himself in the crude arts of the pressure cooker bomb through the internet.

According to accounts of the surviving brother's responses to questioning, he has insisted that they acted alone and that he knew of no other plots or undetonated bombs. Two US officials involved in interrogations of the suspect told the Associated Press that preliminary evidence was that there were no ties to any Islamist terrorist group.

Sageman said that contrary to the popular perception that most terror attacks are orchestrated and organised, in fact the lone wolf attacker is the contemporary norm. "The huge majority – 90% – of plots in the west since the 1990s have come from people who decide to do it on their own and who don't have links with the outside."

Christopher Swift, an adjunct professor of national security studies at Georgetown University, sees another common feature in Tamerlan Tsarnaev's profile: his feelings of alienation as someone caught between two worlds, the Central Asia of his birth (he was born into an ethnically Chechen family in Kyrgyzstan on 21 October 1986) and the US where he had lived for the past decade. On 5 September last year he applied for US citizenship, yet seven months later he carried out one of the worst terror attacks on US soil since 9/11.

"In many ways, Tsarnaev's story was similar to that of the 2005 London bombers: he was married, educated, had a child; he didn't feel like his life was going anywhere and was looking for something that would give it meaning, and in the course of that he went down a rabbit hole of Salafist jihadi ideology," Swift said.

'Everything in the will of God'

Tsarnaev's self-radicalisation appears to have begun around 2009 and 2010. Before that he displayed the outward characteristics of a secular immigrant to the US – he was a boxing champion who seemed to do well at Cambridge Rindge and Latin high school, though he said in one interview that he had no American friends.

Around 2009 it all started to change. His uncle Ruslan Tsarni, who lives in Maryland, said that about that time his nephew started to speak about "God's business" and putting "everything in the will of God". He gave up boxing and drinking, and for a brief period began wearing long white linen garments, a neighbour told the Boston Globe.

At about the same time, people who knew Tsarnaev's mother, Zubeidat Tsarnaeva, witnessed a similar transformation in her. Alyssa Kilzer, who used to go to the Tsarnaev house in Cambridge to have facials from Zubeidat, wrote in a blogpost that in about 2010 she noticed the mother wearing the hijab for the first time.

Zubeidat started invoking Allah and the Qur'an, and began fasting. During one facial session, she recited a conspiracy theory to Kilzer that the attacks had been concocted by the US government to instill hatred towards Muslims – a theory she said her son had taught her.

By early 2011, the transformation in Tamerlan Tsarnaev was so marked that it was picked up by the Russian security services who approached the FBI asking for information on him. The FSB said "he was a follower of radical Islam and a strong believer, and that he had changed drastically since 2010".

So the question remains: why did Tamerlan Tsarnaev become radicalised, and did anyone lure him down that path? Counter-terrorism investigators, seeking clues behind terror attacks, routinely look for evidence of the influence of an extremist cleric or ideologue. For instance, Major Nidal Malik Hasan, the military psychiatrist accused of killing 13 people in the 2009 Fort Hood shooting, had communications with Anwar al-Awlaki, the American imam later killed by a CIA drone strike in Yemen.

By contrast, the evidence that Tsarnaev received formal radicalisation is scant at best. Time magazine has reported that the Russian suspicions about him were based on his attendance of a radical mosque in Kotrova Street in Makhachkala, the capital of Dagestan, while visiting his parents in 2011 and again during the six months he spent in Dagestan between January and July last year. But there is no suggestion of any determined ideological grooming.

Forced to leave the mosque

Back at home in Cambridge, Massachusetts, the signs of any religious incitement are even more threadbare. Tsarni, the uncle, said he believes his nephew came into contact with a person of Armenian descent who "brainwashed him", but no such acquaintance has been identified or tracked down.

The mosque in Cambridge where Tsarnaev occasionally prayed, the Islamic Society of Boston in Prospect Street, is an American-Islamic place of worship that prides itself on its moderate theology. "Our mosque is one filled with attendees who are teachers, businessmen, doctors and lawyers, all of whom are committed to the public good," it said in a statement.

The influence that the ISB appeared to have had on Tamerlan Tsarnaev was to try and reign in his more radical Islamist views. On 16 November 2012, he stood up and interjected during an address in which the immam argued it was appropriate for members of the mosque to celebrate the upcoming American national holiday of Thanksgiving just like the birthday of the Prophet Muhammad; Tsarnaev objected that celebration of secular holidays was forbidden in the faith.

Following a second outburst on 18 January this year, Tsarnaev was forced to leave the mosque after others in the congregation shouted out "Leave now!" Tsarnaev had again stood up, this time calling the preacher a "hypocrite" and accusing him of "contaminating people's mind" by declaring that Martin Luther King was a great person.

After that event, leaders of the Cambridge mosque issued Tsarnaev with an ultimatum: either stop the interruptions and remain silent, or never come again. "While he continued to attend some of the congregational prayers after the January incident, he neither interrupted another sermon nor did he cause any other disturbance," the statement says.

If there appears to have been no charismatic religious figure behind Tsarnaev's radicalisation, then what did take him down this "rabbit hole"? There is one important clue in the account given by Alyssa Kilzer of the conversation she had during that facial session with the bombing suspect's mother, Zubeidat.

When Zubeidat claimed that the 9/11 attacks were an anti-Muslim plot, she said: "My son knows all about it." Then she added: "You can read on the internet."

The schematic information that has been gleaned so far about Tsarnaev's actions and motivations suggest that the most important single influence on him was his own exploration of the internet. A YouTube page under his name shows that he was clearly interested in learning more about Islamic theology, with accounts on it from young men describing their conversion to the faith. About four months ago Tsarnaev "liked" a recorded sermon by an Australian sheikh called Feiz Mohammed in which the Sydney-based sheikh decries Harry Potter, ranting that that the children's story "glorifies paganism and evil".

Richard Nielsen, a graduate fellow at the Harvard Academy for International and Area Studies, has looked closely at the YouTube page and said what strikes him about it is its emphasis on doctrinal issues. "A number of the videos are not what a casual observer might think of as 'Islamist terrorism' videos. Rather, they are doctrinal videos debating precise points of doctrine about how God deserves to be worshipped."

There is nothing inherent in Islam that encourages terror attacks, Nielsen said. But it does appear that a particular interpretation of Islamic doctrine was important to Tsarnaev and that "his perception of doctrine was part of his motivation".

Two videos that have been removed from the page were filed under a subsection called "Terrorism". Mark Rasch, former head of the Justice Department's computer crime unit, said that it should not be difficult for federal investigators to retrieve the binned files.

"The odds are pretty good they will be able to pull this stuff up. Tsarnaev doesn't seem to have made much of an effort to conceal what he was doing on the internet," he said.

Marc Sageman points to another line of inquiry that Tsarnaev was pursuing judging from his surfing of the web: an interest in Chechen nationalism and the abuses Russia was committing in the republic that he then appears to have equated with US operations in Afghanistan. "These jihadi websites connect what Russia is doing in Chechnya with what America is doing in Afghanistan. There is a migration from anti-Russian feeling to anti-western."

There are echoes of that point in what US officials who have interviewed Tsarnaev's younger brother Dzhokhar in hospital have told the Washington Post. The surviving suspect of the marathon bombings indicated to them that the wars in Afghanistan and Iraq were important motivations.

Amid the mounting evidence that Tamerlan Tsarnaev led himself down the "rabbit hole" of self-radicalisation, with the help of material he found, again for himself, on the internet, the key mystery still continues to ellude experts and investigators: why he chose to convert his increasingly intense religious beliefs into violent actions. It is one thing to be a fervent believer, another entirely to be a bomber.

The clues here are at this stage in the investigation virtually non-existent. When the FBI looked into Tsarnaev in 2011 at the request of the Russian FSB, including in their efforts a full scan of his internet habits and interviews with Tamerlan himself and his relatives, they found no "terrorism activity, domestic or foreign".

That raises the ultimate challenge for the US and other western powers: the growing suspicion that Tamerlan Tsarnaev decided to place a large black knapsack on his back and plant it close to the finishing line of the marathon, knowing it would sever lives and limbs, purely for his own reasons.

"There is any number of paths towards violence, and this seems to have been their own," Sageman said. "Two young people dreaming dreams of glory, with a sense of moral outrage, looking for identity, and extremely difficult to detect."

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