WASHINGTON — In March 2011, the Russian security service sent a stark warning to the F.B.I., reporting that Tamerlan Tsarnaev was “a follower of radical Islam” who had “changed drastically since 2010” and was preparing to travel to Russia’s turbulent Caucasus to connect with underground militant groups. Six months later, Russia sent the same warning to the C.I.A.

On April 15, law enforcement officials say, Mr. Tsarnaev and his younger brother, Dzhokhar, set off bombs at the Boston Marathon, killing three people and maiming many others.

The Russian warnings to the F.B.I. and the C.I.A. also raised questions about Mr. Tsarnaev’s mother, Zubeidat, according to two senior American officials. The Russians were most concerned about Mr. Tsarnaev because they had information that he planned to travel to Russia, according to one of the officials. “The Russians were concerned that mother and son were very religious and strong believers, and they could be militants if they returned to Russia,” the other official said.

The mother, the officials said, did not fit the profile of a potential extremist, leading American counterterrorism officials to not express much concern about her. They did not set up a travel alert on her, for instance, one of the officials said. But they nonetheless added her name to the Terrorist Identities Datamart Environment, or TIDE database, when Tamerlan’s name was added to it in October 2011, the official said.