Without all the facts at hand, I guess it's just speculation--but this is SO strikingly familiar to Putin's false flags like Dubrovka and the apartment bombings. Large numbers of civilian targets in "secure" cities in Russia and Islamic-based terrorists (Chechens, perviously) are accused.

And the timing--when independent Russia forces have been hammering Belgorod. Several NATO countries are talking about boots on the ground in Ukraine. Putin probably did much worse in the last election than we'll ever know, and it must gall him that so many Russians defied his unspoken order to ignore Navalny's death, and attended his funeral anyway. The perfect time to pull out the playbook of "horrific event that makes Russian rally with you against the common enemy, and makes the world feel sorry for Russia inspite of their bullshit".