They come from a very Islamic country and it's been said that they'd recently started wearing traditional Muslim clothing--a switch from the european clothing they'd been wearing for most of their lives. It's easy for kids to become obsessed with a cause... even if it's a terrible one. Basically, all signs point to this being just another religious extremist attack.
It appears that the elder brother got disillusioned/pissed-off (I guess he came here in his late teens, so didn't get the chance to adjust). He got angry. Maybe he felt that he belonged in the Olympic team and didn't make it. Whatever. But this provided the kindling; and some religious nut turned this anger into a weapon.
The younger brother was just too much under the influence of the elder, it appears from what I've read. Maybe he looked up to him, and just followed him blindly. Who knows.
As long as we keep figuring out ways to hate each other, we'll keep figuring out ways to kill each other.
The big question is to what degree it was organized, or whether the older brother's social problems (inability to connect etc.) so alienated him that he just seized on the most empowering role model he could think of, an Islamic rebel. Chechnya is a country that has been ruled pretty poorly by Russia at it's not so long ago that Chechen Muslims were seen as heroic freedom fighters in the west.
In short, it's unclear whether they were actually involved with any organized jihad groups or just developed a personal obsession with the ideas and copied the methods.
