The unidentified law enforcement source told The New York Times Russian authorities indeed tipped the FBI and the CIA in 2011 that Tsarnaev might be worth monitoring; however, agents found no evidence of a possible crime when they interviewed him.
Tsarnaev, 26, was killed this month by police in suburban Boston after he and his brother, Dzhokhar, 19, allegedly planted two backpack bombs that went off at the marathon finish line, killing three people and injuring more than 260.
The younger brother was later seriously wounded and captured. CNN said he was moved this weekend to a federal prison hospital and has become less cooperative with U.S. investigators since he was charged criminally and advised of his Miranda rights.
The Russians never responded to requests from Washington for more information on why they considered Tamerlan Tsarnaev to be a potential threat, the source said. Russian officials only recently said they intercepted two telephone calls involving Tsarnaev's mother allegedly discussing so-called jihad.
The Tsarnaevs are ethnic Chechens, who have been locked in a long struggle for independence from Russia. The Times said the connection may not have raised any major red flags because the Muslim insurgents have been focused on their conflict with Russia rather than the idea of a worldwide Muslim holy war.