It looks like a Russian false flag operation to me.
Would Russia willingly allow an operation to kill so many precious innocent civilians in some false flag attempt? Who benefits from this attack and loss of life there? For it to have been Russia itself behind it, what is Russia today better positioned to do than they were before this attack? Anything?
It doesn’t feel like the old ISIS but is ISIS still the same rough and tumble band of ideological killers that they were before? They’ve been relatively quiet internationally, lately.
They need to kill concert goers when they're already winning.