I'm not sure how we're ever going to be certain of exactly who did it. If it was the Russian state behind it, as seems most likely, they are never in a million years going to admit it.
According to experts on these matters, it would be virtually impossible to manufacture any such substance without purpose-built, specialist facilities, otherwise anyone trying would almost certainly end up killing themselves in the process.
So it does point to state-sponsored action, and you do have to wonder who else but the Russian state would want to kill its own traitors?
Though of course (as I've seen on another forum lately) the conspiracy-theorists are all charging out of the woodwork, saying of course it was the CIA...
I'm not sure Putin would be at all bothered if we kicked all the oligarchs out, either - aren't many of them only here in the first place because already they were not on his Christmas card list?