The F.B.I. has concluded that there was little its agents could have done to prevent the Boston Marathon bombings, according to law enforcement officials, rejecting criticism that it could have better monitored one of the suspects before the attack.

That conclusion is based on several internal reviews that examined how the bureau handled a request from a Russian intelligence agency in 2011 to investigate whether one of the suspects, Tamerlan Tsarnaev, had been radicalized during his time in the United States.
F.B.I. Said to Conclude It Could Not Have Averted Boston Attack -

It would be interesting to know what information the N.S.A. had on Tamerlan, considering the dragnet on phone and Internet records they conduct.