A growing trove of evidence, both online and offered by relatives, shows similar elements in the experience of the Boston bombing suspects, especially 26 year-old Tamerlan, who relatives said grew more strident over a two-year span before the attack. He was killed in a shootout with police on April 19.
Tsarnaev told investigators the brothers found bomb-making information in the pages of Inspire, an online magazine affiliated with al-Qaida, said Rep. C.A. "Dutch" Ruppersberger, D-Md. and the top Democrat on the House Intelligence Committee.
A U.S. official briefed on the interrogation confirmed that wars in Iraq and Afghanistan were given as a motivating factor behind the Boston attack.
"Everything that I see right now seems like they were radicalized through the Internet," Ruppersberger said.
That process appeared to be a gradual one. Tamerlan, who at one point had hoped to be an Olympic boxer, gave up sports and began encouraging his mother and his wife to wear a headscarf in public.
His YouTube channel, started in August 2012, contains "likes" of an interview about Russians converting to Islam and a recording of an interview with a famous sheik.
The page also links to a video entitled "The Black Flags from Khorasan," which refers to a prophecy of Muhammad predicting the rise of an army from the Central Caucasus that will march to Jerusalem. There are also videos of a well-known Russian singer named Timur Mucuraev, who in his music advocates jihad, according to Andrei Soldatov, a Russian researcher and expert in the country's intelligence services.
Soldatov says he believes the brothers' online interactions first brought the suspects to the attention of Russian authorities, who asked the FBI to interview Tamerlan before a trip he took back to Russia in 2012, citing his growing militancy.