Ah yes, that also explains why the Russians specifically warned us about them repeatedly.
Who made this allegation? Nobody knows. They chose to remain anonymous.
The FBI said they were asked for information about Tamerlane Tsarnaev in 2011 by a foreign government, but when the FBI asked for more information about him, they did not receive it.
Why not?
Tsarnaev traveled to Russia in 2012. If the Russia knew he was an extremist, why did they allow him to travel to and from Russia?