Tamerlan Tsarnaev, who died during a shootout with police several days after the twin bombings, subscribed to publications that promoted white supremacy and government conspiracy theories, as well as read material on mass killings, the BBC said.
Tamerlan Tsarnaev, 26, and his brother, Dzhokhar, 19, had been perceived as self-styled radical jihadists. The BBC said it spent months talking to the brothers' friends to try to understand what led to their radicalization.
Three people died and 264 people were injured when the two bombs exploded near the finish line April 15.
The Tsarnaev brothers, ethnic Chechens, moved about Russia's troubled region -- torn apart by a violent Islamic insurgency -- during their early years, but spent the last 10 years of their lives in Cambridge, near Boston.
The brothers' friends told the BBC Tamerlan Tsarnaev became passionate about Islam after he was frustrated when his boxing career faltered because he did not have U.S. citizenship.
One friend said Tamerlan Tsarnaev "just didn't like America. He felt like America was just basically attacking all Middle Eastern countries."
Another friend said the elder brother latched onto Islam and was a "Muslim of convenience."
Dzhokhar Tsarnaev, who has been charged and is in custody for the bombings, had been reading militant Islamic websites before the bombings, the BBC said.
Friends said the younger brother smoked marijuana and rarely prayed.
One friend told the BBC Dzhokhar Tsarnaev was dominated by his older brother, who didn't approve of his "party lifestyle."
"He [Dzhokhar] was intimidated. That would probably be the best word," the friend said. "He took him very seriously. He was an authority."