FBI spokesman Michael Kortan said Tuesday's visit was productive and that Mueller is now on his way to other overseas destinations. Kortan declined to elaborate on conversations in Moscow about the Boston bombing probe.
The U.S. and Russia have been collaborating on the criminal investigation into the two suspects in the Boston case.
Russian agents placed the elder Boston bombing suspect, Tamerlan Tsarnaev, under surveillance during a six-month visit to southern Russia last year, then scrambled to find him when he suddenly disappeared after police killed a Canadian jihadist, a Russian security official has told The Associated Press.
U.S. law enforcement offic6ials have been trying to determine whether Tamerlan Tsarnaev was indoctrinated or trained by militants during his visit to Dagestan, a Caspian Sea province that has become the center of a simmering Islamic insurgency.
The Russian security official with the Anti-Extremism Center, a federal agency under Russia's Interior Ministry, told the AP that Russian agents were watching Tsarnaev, and that they searched for him when he disappeared two days after the July 2012 death of the Canadian man, William Plotnikov, who had joined the Islamic insurgency in the region. The Russian official spoke only on condition of anonymity because he was not authorized to speak to the news media.
Associated Press writers Arsen Mollyev and Lynn Berry in Moscow contributed to this report.