According to the U.S. security official, the CIA nominated Tsarnaev's name "out of an abundance of caution" for inclusion in the U.S. government's terror watch list known as TIDE, the Terrorist Identities Datamart Environment. Maintained by the National Counterterrorism Center, the database of more than half a million names is the base point for relevant federal agencies like the Transportation Security Administration to produce no-fly lists.
When Tsarnaev returned to the United States in June 2012, there was no "ping" of his TECS database entry because it had already expired. But there was also no positive hit in the TIDE database because it turned out that all of the personal details provided to the CIA by Russia were inaccurate.
According to the U.S. security official, even if Tsarnaev's name and details had been accurately provided into the TIDES database, U.S. customs officials would have found that the FBI's investigation had been closed and he had not been determined to be a threat.
Furthermore, even if the TECS database entry for Tsarnaev had not expired after a year, that, too, would have brought up the information that his case had been closed.