Four the fourth time in a few years a double agent has been prisoned in England. In every case it was a double agent of Russian origin and in this case was released eight years ao by Russia in a spy swap after being imprisoned for treason. Double agents don't seem to fare too well in spy stories and it does appear it's not a safe retirement in England and somebody has killed four of them now - three actually Skirpol did not die, he was merely harmed as was his daughter, a Russian national visiting her father after she expressed concern about his mental health.
PM May announced it was a Russian plot, Boris Johnson, foreign minister claimed it was ordered by Putin. When asked for evidence and for samples a demanded by international protocol, ratified by the UK that governs behavior when bioterror arms are found, they declined. As of this writing zero evidence of any kind, medical or otherwise, has been offered.
In 1999 the New York Times reported the Pentagon had begun a joint US Uzbek project to deconstruction Soviet era nerve has production and claimed the plant had something to do with the Nerve gas alleged by PM May to have been used, the problem being it works 3-5 times faster, leaves yellow stain and would have killed everybody in the room had it actually been used. As it was Skirpol and his daughter walked around a bit and were the only ones that became unconscious some time later. Bad food could have done this frankly.
When Ishii unleashed Sarin a subway in Japan the body count was non-trivial. The agent alleged to have been used in the Skirpol would have killed everyone in the restaurant, being 3-5 times more potent. It also leaves a nasty yellow stain and smells awful. If it had actually been used that building would have been encased in concrete by now by international convention.
One might postulate infinitesimal amounts may have been used. Hence the importance of a sample.
When compared against the samples the Americans took from Uzbekistan in 1999 they may in fact prove to be the same, but we would have to rely on the honesty of the Americans to produce uncorrupted evince and if they have complicity in the Skirpol affair this is not a valid assumption.
Russia per se did not have much say in this, they let the US dispose of the Soviet era nerve gases as they had requested. A prudent policy as it lets the Russians say "if there's a problem with that stuff look to the US, they have it now, not us".
Of course knowing what it is and where it came from does not tell you the who and why of this. Causing harm to a Russian national on English soil in a double agent cock-up.
Neither nation is interested in Skripol, the Russias gave him away in a spy swap while the British cared to much they failed to protect him from something and won't say what. Or why.
It is not expected they'll ever say. Brexit negotiations in Europe where PM May has to negotiate the exit price ar scheduled mere days after the Skirpol incident Russia and PM May are on the same side there.