Most of Russian opposition does support this theory. Many people do, in general. It's hard to call it a conspiracy theory when they literally caught a couple of FSB guys loading bags of explosives into an apartment building. The official version is that this was a "security training", but c'mon.
> A conspiracy theory is an explanation for an event or situation that asserts the existence of a conspiracy (generally by powerful sinister groups, often political in motivation),[3][4][5] when other explanations are more probable.
Evidently, different people assign different probability to security chief thinking “while real bombings are actually exploding each week and whole cities sleep outside to avoid death, let’s plant a fake bomb and see what happens”
FWIW there was a public opinion poll by Levada back in 2002 about this. 6% said FSB did it, 37% said that there's no clear evidence but they could have done it, and 38% said that it's definitely not the feds.
