The Russian intelligence agency warned US authorities twice that Tsarnaev was a radical Islamist and potentially dangerous. As a result, Tsarnaev was entered into two US government databases: the Terrorist Identities Datamart Environment and the Treasury Enforcement Communications System (TECS), an interagency border inspection database.
A special note was added to TECS in October of 2011 requiring a mandatory search and detention of Tsarnaev if he left the country. "Detain isolated and immediately call the lookout duty officer," the note reportedly said. "Call is mandatory whether or not the officer believes there is an exact match."
"Detain isolated and immediately call the lookout duty officer."
Unfortunately, Tsarnaev's name was not an exact match: it was misspelled by one letter. Whoever entered it in the database spelled it as "Tsarnayev." When Tsarnaev flew to Russia in January of 2012 on his way to terrorist training, the system was alerted but the mandatory detention was not triggered. Because officers did not realize Tsarnaev was a high-priority target, he was allowed to travel without questioning.