The Russian government warned the U.S. that Boston Marathon bomber Tamerlan Tsarnaev was a radical Islamist, but authorities failed to detain him as he traveled back and forth from the Russian republic Dagestan for terror training, according to a Congressional report.

One time, authorities were supposed to pull him aside for questioning at JFK airport in New York City because he was possibly armed and dangerous, but he slipped away because someone had misspelled his last name in a security database.

The warnings began in 2011, when the Russian intelligence agency warned the FBI that Tsarnaev was once associated with violent Islamists.

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