That the Russians wouldnt admit even if it was them, thats fairly clear. However, these sorts of situations are way too obvious even for them. They are well aware that if certain person, obvious opponent to the Russian state, dies people can add 2+2 and see who got the biggest motives. Now thats a pure speculation and its going a bit too much into detail but here we got two possible cases of 1) Its the Russian government and they dont give a toss about who may think what and 2) Its not them and some of their enemies are trying influence the world opinion towards it(this may well include oligarchs who simply want the current status quo to remain).