A few days ago, the New York Times reported on the FBI excusing itself for not preventing the Boston bombings:
WASHINGTON — 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.
And why couldn't the FBI—with its massive surveillance powers and partnership with the NSA—have "better monitored" Tamerlan Tsarnaev? Because it doesn't have enough surveillance powers!
F.B.I. officials have concluded that the agents who conducted the investigation and ultimately told the Russians that there was no evidence that Mr. Tsarnaev had become radicalized were constrained from conducting a more extensive investigation because of federal laws and Justice Department protocols.
Expect to see this argument elaborated in the future: "Think we've got so much secret surveillance power that we're violating the Fourth Amendment on an hourly basis? Think again! The truth is, we've got so little secret surveillance power that we can't protect you!"
The FBI, according to the article, has refused three requests to testify before the House Homeland Security Committee to testify about the attack, saying it does not comment on open investigations.
At any rate, the issue isn't whether the FBI could have prevented the bombings or not—predicting crimes, no matter what one's surveillance resources, is dicey business. The issue is how they're framing the violence to say that they maybe could have prevented the attack, if only they had more surveillance powers.