Russia warned the U.S.

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Russian intelligence agency FSB cabled the FBI about its concerns in March 2011, warning that Tsarnaev was known to have associated with militant Islamists.

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In September 2011, the FSB sent a cable to the CIA, restating the warnings of the first memo. NBC News quoted sources close to the congressional investigation as saying a second note about Tsarnaev was entered into the TECS system the next month, but spelled his name "Tsarnayev."

The note directed that if Tsarnaev were encountered leaving or re-entering the United States, his detention was "mandatory."

In January 2012, Tsarnaev went to JFK airport in New York to board a flight to Moscow, triggering an alert. But he was not considered high priority among the 100 other names on a "Hot List" of people traveling through JFK that day, NBC News said, citing sources familiar with the report.

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U.S. officials have said a misspelling of Tsarnaev's name on flight records may have contributed to some law enforcement agencies not being alerted to his movements.

Several days after the April 15, 2013, Boston Marathon bombings that killed three people and wounded more than 260, Tsarnaev died after a gunfight with police while he and brother Dzhokhar, now 20, were trying to flee the city.

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NBC News quoted a congressional staffer as saying: "The report is not blaming the FBI," but "looking at processes and filling holes."

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