Still haven't seen anything to explain how a truck bomb laden with explosives managed to end up travelling on the bridge from Russia.
Seems very likely there are Ukrainian SOF/partisans/sympathisers etc.. operating in Russia. I mean there are a lot of Russian-speaking Ukrainians, Ukrainians with relatives in Russia etc. or who are living in Russia themselves.
Quite difficult for Russia as some other "Ukrainians" basically see themselves as ethnically Russian and are enlisted in the Russian army or Navy and indeed are seen by Russia as ethnically Russian... so the Russian-speaking Ukrainians loyal to Ukraine can fit into Russian society quite easily.