islamic terrorists from one of the Caucasus "republics"
disgruntled vets
separatists
FSB agents
I'd figure it's some IS offshoot, although regardless of who actually did it the FSB will likely blame it on the Chechens as is tradition. Especially if they did it themselves
except these Nazis are also somehow Jewish, apparently
The guardian did an interesting piece on that:
The form of Russian fascism Dugin and Prokhanov defended is like the central versions of European fascism – explicitly antisemitic. As Snyder writes, “… if Prokhanov had a core belief, it was the endless struggle of the empty and abstract sea-people against the hearty and righteous land-people. Like Adolf Hitler, Prokhanov blamed world Jewry for inventing the ideas that enslaved his homeland. He also blamed them for the Holocaust.” ... . ... By claiming that the aim of the invasion is to “denazify” Ukraine, Putin appeals to the myths of contemporary eastern European antisemitism – that a global cabal of Jews were (and are) the real agents of violence against Russian Christians and the real victims of the Nazis were not the Jews, but rather this group. Russian Christians are targets of a conspiracy by a global elite, who, using the vocabulary of liberal democracy and human rights, attack the Christian faith and the Russian nation. Putin’s propaganda is not aimed at an obviously skeptical west, but rather appeals domestically to this strain of Christian nationalism.