What I have read about T-brothers makes me think that T Jr. is a close to 100% product of USA culture. He spent half of his life (from 10 to 19 years) in USA and was integrated well into society. And I tend to think that Tamerlan as well. USA provided them with some level of life and ambitions to be "champions" but not the proper ideology to put ambitions to good so they selected radical islam. A guy of non-islamic origin would go to nearby school and kill some children.

Muslim terrorists/guerilla in Russia usually have rather different reasons and different way.