Where did the Ukies get the intel?
Probably from Russians.
People think there is some solid line of national, social, and cultural separation between Ukraine and Russia. There isn't. These people are ethnically related and there are Ukrainians that are staunchly pro-Russian and Russians who are staunchly pro-Ukraine.
Because, really, this is a civil war.
The Ukrainians do not have the tactical capability to carry out such a car bombing in Russia.
The CIA has the capability but would not in Russia.
In my opinion that makes the most likely perpetrator Russia’s FSB(successor to the KGB).
I recall that the assassination of the original Rasputin was carried out by noblemen loyal to the Czar, who wanted to get rid of a “bad influence”. History doesn’t repeat, but sometimes it rhymes.