Facial-recognition software did not identify the men in the baseball caps, according to Davis, who said that technology came up empty even though both Tsarnaevs' images exist in official databases: Dzhokhar had a Massachusetts driver's license, the brothers had legally immigrated, and Tamerlan had already been the subject of some FBI investigation.
The FBI had had contact with Tamerlan at least as far back as 2011. Tamerlan, whose ethnic Chechen family immigrated to the United States in 2002, had indicated his interest in radical Muslim ideology both on Internet ramblings and with family and friends. On YouTube, Tamerlan created a playlist of videos titled "Terrorists."
The brothers' mother, Zubeidat Tsarnaeva, told Russia Today television on Friday that Tamerlan "got involved in religious politics five years ago." She said FBI agents had been watching her son for "three to five years. They knew what my son was doing. . . . They used to tell me that he was really a serious leader and they are afraid of him."
The FBI said Friday that at the request of a foreign government that was concerned that Tamerlan "was a follower of radical Islam and a strong believer," the agency had interviewed him and his relatives but "did not find any terrorism activity, domestic or foreign." Officials have acknowledged that the request came from Russia.
Once the photos of the men in caps were made public Thursday, the FBI tip line filled with calls, including one from the brothers' aunt, who provided her nephews' identity, according to federal law enforcement officials.
As investigators expected, making the photos public not only brought in new information, but also spurred the brothers into action.
On Thursday evening, police responding to a robbery at a 7-Eleven in Cambridge, Mass. examined surveillance video and noticed that in addition to the robber, the convenience store had been visited that night by two men who looked like the bombing suspects.