A Canadian boxer who was killed while fighting with jihadists in Russia has emerged as a key contact who may have set the elder Boston bomber on his path to violent extremism.
In what could be a breakthrough in the attempt to understand how Tamerlan Tsarnaev - himself a skilled boxer - became radicalised and turned to violence, Moscow's respected Novaya Gazeta newspaper revealed his links with William Plotnikov, who was killed in a battle with security forces in the southern Russian republic of Dagestan last year.
During his visit to Dagestan last year, Tsarnaev also met on several occasions a terrorist of mixed Dagestani and Palestinian parentage, who was being closely watched by the Russian security services. That man, Makhmud Mansur Nidal, had been under surveillance for six months as a suspected recruiter for Islamist insurgents fighting Moscow's rule in the region.
Tsarnaev, 26, died during a shoot-out with US police in Boston and his brother Dkhokhar, 19, was captured.
In the investigations that followed there have been few clues as to who played the key roles in radicalising Tamerlan, who is thought to have taken the lead in instigating the Boston attacks. The new claims have come from security officials in Makhachkala, the Dagestan capital, where the brothers' parents moved from the US and where Tsarnaev spent six months last year.
According to the report in Novaya Gazeta, Tsarnaev came on to the radar of Dagestan's anti-extremism unit when he was seen "more than once" with Nidal, 19. A month later, Nidal was killed after he blockaded himself in a house with weapons. He had been accused of being part of a rebel group that organised a twin bomb attack in Makhachkala, killing 13 people.
Russian security operatives found Tsarnaev had been linked to William Plotnikov, an ethnic Russian citizen of Canada, whom they had interrogated in 2010 after he arrived in Dagestan, ostensibly to study Islam. Plotnikov gave a list of people in Europe and the US with roots in Russia's North Caucasus, with whom he had communicated via online social networking sites. Among those whose names Plotnikov volunteered was Tamerlan.
Plotnikov was released, but by 2012 he had joined Dagestan's insurgents - living in forest camps where he was known by fellow insurgents as "the Canadian". He was shot dead, aged 23, in a battle with Russian security forces in July last year that left six other militants dead.
It is unclear whether Tsarnaev and Plotnikov met through boxing circles or only communicated online, but their life paths suggest such a meeting was possible. Both were keen amateur boxers with roots in Russia who turned to Islam after finding it hard to adapt in their adoptive countries. Tsarnaev also visited his aunt in Toronto, where Plotnikov lived with his parents. Novaya Gazeta's security source said the men communicated online via a site associated with a non-governmental organisation called the World Assembly of Muslim Youth. "We pay special attention to foreign or ethnic Russian converts," said the source. "They are extremely ideological and psychologically vulnerable; they're more easily persuaded to do anything."