That's insane. That little old man in Moscow really feels like he can do anything, doesn't he. I'm obviously speculating, but really, who else could it be?
Well despite being few in numbers, false flags do happen across the globe.
Given the dosage was survivable, it isn't far from imagination to also conclude that someone else perpetrated the action knowing full well the obvious blame would fall at Russia's feet (who, of course, do have history with this sort of thing.)
Seeing how most jump to the same conclusion, it's obviously a good tactic.