That announcement was made by an ex-MP who fled to the US after becoming the object of a criminal investigation in Russia. It is very likely that he is some kind of third rate intelligence asset trying to muddy the waters. Going off of the evidence available right now, I think Azov are the most likely culprits. I suspect that the assassin acted without the knowledge of the Azov leadership, however - since a large portion of the battalion is in Russian captivity right now, I do not think the leadership cadre of the battalion would be stupid enough to greenlight something like this. She may have received some support from the CIA, though, but without any new evidence that is just speculation.