The Russian government warned the U.S. that Boston Marathon bomber Tamerlan Tsarnaev was a violent radical Islamist more than a year and a half before the April 2013 bombing. But Tsarnaev's name was misspelled in a security database, leading authorities to miss him completely.
According to NBC News:
In March 2011, the FSB, the Russian intelligence agency that arose from the remnants of the KGB, sent a cable to the FBI with its concerns about Tamerlan Tsarnaev and the Tsarnaev family.
The letter was a page and a quarter long and very detailed. It included contact information, with addresses and phone numbers, for many of the members of the Tsarnaev family, including Tamerlan and his mother. It warned that Tamerlan was known to have associated with violent radical Islamists, including a Canadian Muslim convert named William Plotnikov who was later killed while fighting for the mujahideen in Dagestan.
The FBI responded to the FSB cable by opening an investigation of Tamerlan Tsarnaev in March 2011.
In June 2011, the FBI investigation of Tamerlan Tsarnaev was closed. According to the report, "the assessment found no links to terrorism."