After Russian officials informed the FBI in 2011 that Tsarnaev "was a radical follower of Islam and a strong believer" and had "changed drastically" prior to departing the U.S. for Russia, the FBI investigated. However, when the FBI went back to Russia to request more information about Tsarnaev, they declined to divulge any more information.
It was only after the bombing in April 2013 that Russia informed the U.S. about a call between Tsarnaev and his mother in which they discussed "Islamic jihad."
"They found that the Russians did not provide all the information that they had on him back then, and based on everything that was available the F.B.I. did all that it could," a senior American official told the New York Times.
It's still difficult to say whether the information would have helped prevent the attack as the Tsarnaevs left a very small trail and did not have a detectable network. Tamerlan was killed while running from police and his younger brother Dzhokhar Tsarnaev is currently on trial in Boston.
[New York Times]