ISIS just took credit for that. And frankly my first thought was it was Putin false flagging or, if not that, then the Ukrainians had completely lost what was left of their minds.
While I could see ISIS falsely taking credit for it, I'm not seeing the benefit to them in doing something that would result in this not resulting in intensifying the Russia/Ukraine conflict, which I would think they'd want to keep going. So at least on first impression, I'm taking their word for it.
Pretty much Ward Churchill, a nutjob professor and fake Indian, who was ultimately fired for blatant plagiarism.