They are Chechens. HUGE difference. Apparently one too subtle for you to understand (though it shouldn't be). The Chechen Separatists weren't even Islamic radicals until the late 2000s, they were just nationalists.
The American Chechen refugee population despises these two. The Russians and Chechens hate these two. The general response from the average Russian interviewed as a result of this story considers them just some pathetic loser wackjobs who were lashing out because their life wasn't going their way. The two were from a different part of Russia (now Kyrgyzstan), one of them (the younger) never even visited Chechnya, and the other (the older) was quite vocal about his dissatisfaction at not making any friends in the US. That last part is far more in line with the regular pattern of domestic terrorism.
So yes, the Islamist Radicalist angle is a bit overzealous at this point, and is a bit bigoted. The brothers probably sought a connection with that as an excuse, rather than as their driving cause.
I wouldn't be surprised if the Chechen Separatist Movement hates these two, because they have brought the Russian Government's attention back on them.