Who really knows at this point.
It's rather disturbing that these guys could walk in, go full massacre and throw demolition, and be able to leave without getting confronted, save for one guy apparently.
If it is ISIS, (a big if) It would make some sense as ISIS and Chechen rebels were aligned during the Syrian siege. Remember that Chechnya and Dogistan are both fully Muslim provenances with little allegiance to Moscow.
With Russia fully engulfed in the Ukraine conflict, You could see another rise in a Chechnya separatist movement, creating a two front scenario Russia really can't afford ATM.
And you know CIA would love to pull some shit that would make Russia's life miserable.