The Russian government alerted the FBI to suspicions about one of two alleged Boston bombers, but US authorities failed to arrest him because of a spelling mistake.
According to a congressional report for the US House Homeland Security Committee seen by NBC News, Russia's intelligence agency warned US agents about Tamerlan Tsarnaev's links with militant Islamists as early as March 2011.
The FBI opened an investigation, Tsarnaev was interviewed in person and a memo was sent to the US Customs and Border Protection database - TECS - that would trigger an alert every time Tsarnaev left or re-entered the US.
The investigation was closed in June 2011, after the FBI found "no links to terrorism", NBC quotes the report as saying.
In September 2011, Russia's FSB sent a cable to the CIA restating their initial warning, and a second note on Tsarnaev was entered on the TECS system, but his name was misspelled "Tsarnayev".
The note called for his "mandatory" arrest if "encountered entering of exiting the US", warning he could be armed and was dangerous.
On January 2012, Tsarnaev travelled to Moscow en route to Dagestan, where he received terror training for six months, but was not pulled out for a secondary search because his case was not considered high priority among the 100 other names on a "Hot List" of people traveling through JFK that day.
Upon his return in July, Tsarnaev slipped through security at JFK airport in New York because of the misspelling of his name, which failed to trigger an alert, according to the report.
His initial TECS note had expired, but the second, more urgent note, suggesting he could be armed and was dangerous, had not, NBC quotes familiar sources with the report as saying.
Tamerlan Tsarnaev was killed during a shootout with police on 19 April in Watertown, Massachusetts, and his brother Dzhokhar was discovered hiding in a boat later the same day in the wake of the Boston marathon bombings, where three people were killed and 230 were injured.
Dzhokhar Tsarnaev now faces terrorism charges in connection with the bombing that carry a possible death sentence. He has pleaded not guilty.