One of the brothers suspected of carrying out the Boston bombings was in possession of right-wing American literature in the run-up to the attack, BBC Panorama has learnt.
Tamerlan Tsarnaev subscribed to publications espousing white supremacy and government conspiracy theories. He also had reading material on mass killings.
Until now the Tsarnaev brothers were widely perceived as just self-styled radical jihadists. Panorama has spent months speaking exclusively with friends of the bombers to try to understand the roots of their radicalisation.
The programme discovered that Tamerlan Tsarnaev possessed articles which argued that both 9/11 and the 1995 Oklahoma City bombing were government conspiracies. Another in his possession was about "the rape of our gun rights". Reading material he had about white supremacy commented that "Hitler had a point". Tamerlan Tsarnaev also had literature which explored what motivated mass killings and noted how the perpetrators murdered and maimed calmly.
There was also material about US drones killing civilians, and about the plight of those still imprisoned in Guantanamo Bay.
The Tsarnaev brothers, ethnic Chechens, spent their early years moving around a troubled region of Russia torn by a violent Islamic insurgency. But for the last decade they lived in Cambridge, near Boston.
The brothers' friends told us Tamerlan turned against the country and became passionate about Islam after becoming frustrated when his boxing career faltered because he did not have American citizenship. Their friends wouldn't all speak openly because they were afraid of being wrongly viewed as associated with terrorism.
'Mike' spent a lot of time in the brothers' flat.
A spokesperson for Tamerlan's mosque in Cambridge, Nicole Mossalam, said Tamerlan only prayed there occasionally. She portrayed him as an angry young man who latched onto Islam.
Dzhokhar Tsarnaev, Tamerlan's younger brother who has been charged with the bombings, scrawled a note shortly before his capture stating "We Muslims are one body. You hurt one you hurt us all."
The brothers had been reading militant Islamic websites before the bombings.
Friends say the younger brother smoked copious amounts of pot and rarely prayed.
'Tito' told us Dzhokhar's older brother dominated him and didn't approve of his "party lifestyle".
The FBI has been investigating the brothers, and possible connections Tamerlan might have had in the troubled Russian republic of Dagestan which he visited last year. The House Intelligence Committee in Washington is being briefed on his connections.
The committee chairman, Mike Rogers said he believes the brothers' mother, Zubeidat Tsarnaev, was involved in his radicalisation.
"He had family members encouraging, we know that for sure," he said.
Zubeidat denies the allegations.
Tamerlan was killed in April following a gun fight with police which ended when his younger brother ran him over while trying to escape.
Dzhokhar, recently brought to court, denied all charges. If convicted he faces life imprisonment or the death penalty.
You can watch Panorama - The Brothers who Bombed Boston on Monday 5 August at 20:30 BST on BBC One and then on the BBC iPlayer in the UK.