Ukrainian/USA backed terrorists I guarantee.
RT reports at least 40 killed and a hundred injured. Numbers probably going to go up.
And the thing is, this is even more pointless than your average terrorist attack, this is pure blood-lust and desire to butcher Russian civilians KNOWING it won't change anything because Russia has suffered deadlier attacks including on children in the past 25 years and they didn't deter it in Chechnya or any other regions. So even the usual justification for butchering civilians that it will shock the government or people into change is a known factor, a false one with this specific country.
This is just the start. The US is not happy about the results of Ukraine and intends to severely punish the Russian people for choosing Putin again and for withstanding their sanctions.
I also guarantee that the US and west will try their hardest to spin this as a bunch of disgruntled Russians upset about Putin's authoritarian rule and that their weapons, training, idea just poofed out of thin air. Else it could be IS type extremists who are of course a western puppet and would only be in Russia at CIA insistence. They can't allow wholesome bean Ukraine to be associated with this type of behavior despite it being exactly the SBU's modus operandi. Doesn't matter whether the attackers were native Russians or what, guarantee if they aren't IS that they were trained, radicalized, motivated, probably armed by SBU which is a proxy subsidiary of the CIA.