His brother is obviously the ringleader.
Considering the younger one went to school in America, went to prom, had friends, lived in a dorm, went to parties, gained citizenship, etc, while his older brother was investigated by the FBI years prior, wasn't able to gain citizenship, and went back to Chechnya for 6 months where he probably became radicalized and learned how do do all of this stuff... yeah, it's quite obvious.
The younger brother is still just as guilty though. But in life, there are leaders and there are followers.