Yurka, thank you! That really answers many of my questions and confusion over this.

We'll never know who Tamerlan Tsarnaev worked for or not, since they shot him dead.
And the younger son will never get an opportunity to speak.

What drove them in particular will remain a mystery. But you answered many of my questions about radical moslems in Russia.

PressTV has done some really insightful features on moslem converts or "born-again" muslims in Russia; it seems to be quite a movement and some of them seem very sympathetic and they are certainly very intellectual.

For example, here is an interesting documentary: An Iranian woman goes looking for a Russian ex supermodel who converted to Islam [The Heart of Snow - In search of Masha Alalykina]