In January 2011, two counterterrorism agents from the bureau’s Boston field office interviewed Tamerlan and family members, a senior law enforcement official said on Saturday. According to the F.B.I.’s statement, “The F.B.I. did not find any terrorism activity, domestic or foreign,” and conveyed those findings to “the foreign government” by the summer of 2011.

Federal officials said on Saturday that the Department of Homeland Security, however, had decided not to grant a petition from Tamerlan for United States citizenship after officials found a record in his files that he had been interviewed by the F.B.I. His petition was held for further review.

As the law enforcement official put it, “We didn’t find anything on him that was derogatory.”

The Russian state news agency RIA Novosti quoted the father of the Tsarnaev brothers recalling the F.B.I.’s close questioning of his elder son, “two or three times.” He said they had told his son that the questioning “is prophylactic, so that no one sets off bombs on the streets of Boston.”

In an interview in Russia, Zubeidat Tsarnaeva, the mother of the two men, said that the agents had told her that Tamerlan was “an excellent boy,” but “at the same time, they told me he is getting information from really extremist sites, and they are afraid of him.”

After Tamerlan’s visit to Dagestan and Chechnya, signs of alienation emerged. One month after he returned to the United States, a YouTube page that appeared to belong to him was created and featured multiple jihadist videos that he had endorsed in the past six months. One video featured the preaching of Abdul al-Hamid al-Juhani, an important ideologue in Chechnya; another focused on Feiz Mohammad, an extremist Salafi Lebanese preacher based in Australia. He also created a playlist of songs by a Russian musical artist, Timur Mucuraev, one of which promoted jihad, according to the SITE Intelligence Group, which monitors statements by jihadists.

The Boston bombings have led to increased cooperation between Washington and Moscow, a jarring shift coming amid weeks of rancor over American criticism of Russia’s human rights record. Presidents Obama and Vladimir V. Putin spoke by telephone late Friday night, in a conversation initiated by the Russian side, the Kremlin announced. The Kremlin’s statement said both leaders expressed “the building of close coordination between Russian and American intelligence services in the battle with global terrorism.”