I read the entire article on CNN (took me like 10 minutes) but it doesn't mention Islam anywhere? what makes you say it's related?
anyway, this is weird, I still have no clue as to their motive
It seems that the suspects are Chetchen Muslims.
Motive's still a mystery, though the fact that no group has come out and claimed responsibility for the attack is pretty telling - it seems like they were acting alone. Now that one's in custody, hopefully we'll get some answers.