The people that have done the deed where definitly ISIS. But there where many people involved behind the scenes and someone payed for it.

ISIS usually ideologically subverts individuals.

While Ukraine is involved in numerous killings, terrorism and uncountable sabotages inside Russia, (mostly) by either paying or convincing locals.

(I doubt it was the US/ Europe. The CIA/ Europe would have no gain from this.)

But there is no definitive proof (so far) of who actually orchestraded it, so it could be the usual suspect or the obvious one.