Russian authorities warned the United States in 2011 that Boston Marathon bombing suspect Tamerlan Tsarnaev had become: "follower of radical Islam."
The FBI said it was told the now deceased suspect was preparing to join unspecified underground groups.
Tamerlan and Dzhokhar "Johar" Tsarnaev's Islamic beliefs were at the core of their world view, but Tamerlan appears to have been more vocal about his ideology.
The brothers' uncle says Tamerla, 26, would throw out words like "jihad" and "Inshallah" - Arabic for "Allah willing."
Ruslan Tsarni said he began to lose track of his two nephews several years ago when Tamerlan "started carrying all this nonsense associated with religion, with Islamic religion."
When he asked his older nephew why he wasn't in school, he said Tamerlan gave the enigmatic answer: "Oh, I'm in God's business."
A neighbor also reports that Tamerlan recently ranted about Islam and the United States. That neighbor said he had a bizarre encounter with Tamerlan in a pizza shop about three months ago. The older brother argued with him about U.S. foreign policy, the wars in Afghanistan and Iraq, and religion.
He said Tamerlan referred to the Bible as a "cheap copy" of the Koran and that many of this country's wars "are based upon the Bible - how it's an excuse to invade other countries."
"In this case, the older brother is the one that seems to have become religious and drawn to Islam," Criminologist James Alan Fox told the Associated Press. "The older brother dropped out of school ... whereas the younger brother, it was all positives."
But Dzhokhar was not completely silent on Islam. He used social media to express occasional thoughts about his Muslim faith and his dislike of America.
On March 14, 2012, Dzhokhar tweeted: "A decade in America already, I want out." That same day, he added, "I'm trying to grow a beard."
He wrote Nov. 29 that he liked debating people on religion and "crushing their beliefs."
Ironically, on Jan. 15 of this year, Dzhokhar stated: "I don't argue with fools who say Islam is terrorism. It's not worth a thing. Let an idiot remain an idiot."
Then In March, Dzhokhar tweeted: "Evil triumphs when good men do nothing." A week and a half earlier, he reminded his followers, "Never underestimate the rebel with a cause."
The day of the bombing, he wrote: "There are people that know the truth but stay silent and there are people that speak the truth but we don't hear them cuz they're the minority."
The ethnic Chechen family came to this country in 2002, after fleeing troubles in Kyrgyzstan and then Dagestan, a predominantly Muslim republic in Russia's North Caucasus.