Russian authorities warned the FBI in 2011 about Tamerlan Tsarnaev, one of two Chechen brothers accused of carrying out last year's Boston Marathon bombings, but U.S. authorities missed chances to detain him, NBC News reported on Tuesday.

Citing a congressional report it said could be released by the House of Representatives Homeland Security Committee as soon as Thursday, NBC News said the Russian intelligence agency FSB cabled the FBI about its concerns in March 2011, warning that Tsarnaev was known to have associated with militant Islamists.

The network said the FBI opened an investigation of Tsarnaev that month conducted by a joint task force of federal, state and local authorities. Tsarnaev was interviewed in person, and a memo was sent to the Customs and Border Protection database called TECS that would trigger an alert whenever he left or re-entered the United States.

But the investigation was closed in June 2011 after finding Tsarnaev had no links to terrorism, NBC quoted the report as saying.

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.

After spending six months in the Russian region of Dagestan, an experience US investigators suspect played an important role in his radicalization, Tsarnaev flew back to JFK airport on July 17, 2012, but he was not detained or questioned because of the misspelling of his name, NBC News said.

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