You are underestimating the FSB. The attack will have probably been carried out by Muslims as part of some ISIS cell - however unbeknown to them, the orchestrators of this attack (The planners and facilitators ) would have been FSB masquerading as ISIS.
Once the bodies of the terrorist are burned beyond recognition they will be recovered along with planted links back to Ukraine and the blame will be assigned to Ukraine - the next step will be an increased mobilization that will give Putin the 300K troops he needs to wage his next offensive against the city of Kharkiv.
Classic false flag
This maybe all a load of bollocks but it’s my thoughts and I’m sticking with it.
That sort of attack - people doing it genuinely but without knowing who their masters are - is the only way it could be done, but setting that up requires a lot of time and at least as many people in a network as an actual terrorist cell would. The arms would have to be sourced, the attackers recruited, trained, moved about, housed, fed and so on - all with the risk of someone noticing.
It is an awful lot to have to accept when one could just look at the history of these sorts of attacks within Russia, recent information released from the Russian government themselves (never mind the US warning from a couple of weeks ago) and think that a similar group did this.
He absolutely could use this as an excuse to mobilize further mind, even if its just pointing to the public links between Ukraine and Ingushetia independence movements as implying Ukrainian support for this attack.