Agency couldn't find much evidence to support Russian claims against Tamerlan Tsarnaev.
The FBI's internal investigation into the lead-up to the Boston Marathon Bombings concluded that the agency couldn't have done very much to prevent the attacks, according to a report from the New York Times. That's despite a congressional probe into the agency's handling of a security review of Tamerlan Tsarnaev two years before the bombings.
The Russians flagged Tsarnaev for the FBI in 2011, ahead of his trip to Dagestan, Russia, entering him into the massive, complex system used in the U.S. to track and vet terrorism suspects. The FBI interviewed Tsarnaev and his parents, checked his criminal history, his education background, and his web activity. After Russia declined to provide the U.S. with more information on why they'd flagged him as a "follower of radical Islam," the FBI concluded that there was little evidence, based on what it could legally sift through, to support that claim. The New York Times explains that the agency is more or less standing by their work:
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. Agents cannot use surveillance tools like wiretapping for the type of investigation they were conducting.
The officials have also determined that had the agents known that Mr. Tsarnaev had traveled to Russia for months in 2012, they probably would not have investigated him again because there was no new evidence that he had become radicalized.