"White and Muslim"

Some of the things I'm wondering amid all the chatter about the identity of the Boston bombing suspects:

What percentage of Muslims worldwide are "white"? (Although what's white in the USA isn't always white in other places.)

What percentage of Muslims in the USA are white?

Is "white Muslim" an oxymoron to some people?

Why does it matter if the bombers are white?

What, exactly, is an "ethnic Chechnyan"? Does that mean a person from Chechnya is a different ethnicity than a person from, say, Georgia or Azerbaijan? Is this the same thing as an American being a Southerner or a Westerner? 

These are not, for me, rhetorical questions. I really don't know, and I'd like to figure it all out.