This article is terribly misleading about the city. Volgograd is not in the Caucasus at all, nor does it have any affiliation with the unrest in that region. Volgograd is the city formerly known as Stalingrad (from WW2 fame). It is located about the same distance from Chechnya and Dagestan as Sochi is. It’s population is around 90% ethnic Russian, and Muslims make up around 3% of its total population.
Could this have been a Chechen bomb? Absolutely. But Volgograd itself isn’t some Caucasus hotbed of Islamic insurgency like Grozny.
Is there any possibility of this having to do with the movement against immigrants in Russia?