Still, some analysts wonder if a recent row between the two countries over human rights abuses and what Russia interpreted as US interference in Russia’s domestic affairs suggest that lingering suspicions could hamper cooperation on the bombing case.
The FBI declined to discuss any plans to investigate with Russian authorities Tamerlan’s 2012 trip to Russia or other places he visited. But one US official noted that the FBI has a legal attaché in Moscow and one in Kiev, Ukraine, and that one could “safely assume” the investigation is already under way.
The younger Tsarnaev, Dzhokhar, was formally charged Monday with use of a "weapon of mass destruction," specifically an improvised explosive device, resulting in death.
As the 19-year-old recovers in a Boston hospital from throat and leg wounds he sustained in the same shootout, investigators are combing through seized computers, questioning contacts, and revisiting a closed file the FBI had opened on Tamerlan Tsarnaev after Russia asked the US in 2011 to investigate the ethnic Chechen as an adherent of radical Islam and for links to extremist groups.
The FBI closed the file after questioning Tamerlan and family members but finding no evidence of contacts with terrorist organizations.