The Russian warning to the United States government in 2011 about Tamerlan Tsarnaev, one of two brothers accused in the Boston Marathon bombing on April 15, was based on two intercepted telephone calls discussing jihad, both involving his mother, a law enforcement official said on Saturday. The Russian authorities informed the F.B.I. in March 2011 that Mr. Tsarnaev, an ethnic Chechen who had lived in the United States for about a decade, had changed drastically, had adopted extremist views and planned to travel to Russia to meet with underground groups. The F.B.I. sent agents to interview Mr. Tsarnaev and his...