Looks like Chechan origins, but been in US for several years. The younger is 19, and finished school in Boston. Don't know their immigration status.
Wonder what set these two off?
Disaffection.
The older brother arrived after the younger, and was not able to adapt or make friends in the U.S. Resentment built up, he started following Islamist web sites, and basically brainwashed himself. (Not speculation, this is from articles released today.) No doubt the older brother influenced the younger brother.
So, instead of the bomber being a white supremacist, they turned out to be Islamist terrorists. Gee, whoever might have predicted THAT?