Here's a very interesting WSJ post regarding what the dead Boston Marathon bomber/killer did with his six month stay in Russia, Dagestan, and Chechnya. It asked a lot of good questions and offered hints that we might not be at the end of this terrorist saga.
It's a good read. The WSJ is best when it is doing articles like this and not getting into bait-click posts like bashing Apple or Google and mischaracterizing data.
There is currently an American team there to investigate what's going on. However, I doubt we'll learn much unless we receive full cooperation from the Russians. There seems to be attempt by Dagestan and the killer's father to deflect notions that radical elements in central Asia had anything to do with the killer's Islamist indoctrination.