One of the Boston bombing suspects was in possession of extreme right-wing material in the build up to the attack, it has emerged.
A BBC investigation found that Tamerlan Tsarnaev had subscribed to publications detailing government conspiracy theories and white supremacy.
The older of the two brothers, who died after a gunfight with police, had also had reading material on mass killings, the Panorama show found.
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The Tsarnaev brothers have until now been seen as straightforward radical jihadists.
Panorama said it had spent months speaking exclusively with friends of the bombers to try to understand the roots of their radicalisation.
Tamerlan had articles arguing that both 9/11 and the 1995 Oklahoma City bombing were government conspiracies, it is claimed.
Some of the reading material he had about white supremacy commented that "Hitler had a point".
Other literature was about US drone strikes and Guantanamo Bay.
Friends told Panorama Tamerlan had turned against the country when his boxing career faltered.
One man, called 'Mike', who claimed to have spent a lot of time in their flat, said: "He felt like America was just basically attacking all Middle Eastern countries…you know trying to take their oil”
Friends of the younger brother, Dzhokhar Tsarnaev, claimed he smoked 'copious amounts of pot' and rarely prayed.
Panorama: The Brothers who Bombed Boston will be shown on Monday night at 8.30pm on BBC One.