From the AP:
[Tamerlan] Tsarnaev's name was added to a Homeland Security Department database used by U.S. officials at the border to help screen people coming in and out of the U.S. That database is called the Treasury Enforcement Communications System, or TECS.... Three days before he left for Russia, the TECS database generated an alert [a ping] on Tsarnaev. That alert was shared with a Customs and Border Protection officer who is a member of the FBI's Boston joint terrorism task force.U.S. officials told ABC News that Mr. Tsarnaev, DID NOT Set Off a ping in the TECS database, upon his return to the U.S.:
In July 2012, Tsarnaev returned to the U.S., and another alert [ping] was generated in TECS. This information was again shared with the Customs and Border Protection officer on the FBI's Boston joint terrorism task force. But because the FBI had closed its investigation into Tsarnaev a year earlier, there was no reason to be suspicious of his travels to Russia.
From ABC News:
When Tsarnaev returned to the U.S. in June 2012 there was NO "ping" of his TECS entry because it had already expired.