This looks both very serious and very puzzling.

https://en.m.wikipedia.org/wiki/Poisoning_of_Sergei_and_Yulia_Skripal

It is very puzzling because the only possible motive for anyone to use this agent to murder somebody in the UK would be to provoke a crisis in relations between Russia and the West.

The British Prime Minister said:

“Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others“

Both alternatives are possible and both have very serious implications.

But also very serious and very puzzling is the fact that she announced that on Monday 19 March and demanded a response from Russia by midnight on the following day.

Use of nerve gas in an attempt to murder a Russian traitor was suspected immediately following the attack a fortnight earlier, 4 March. There was plenty of time for both British and Russian governments to carefully consider how they would respond.

The British deadline seems intended to provoke the Russian response of demanding that they first be provided with a sample for verification.

Under the relevant international agreements for prohibition and destruction of chemical weapons there are clear procedures for verification by certified laboratories with care taken to preserve the chain of custody of samples for use as evidence of violations. See Section 4, chapter III of the OPCW Blue Book:

Click to access BlueBook.2011.01.pdf

It simply does not make sense that the British government would have set a deadline for response without first providing samples to other laboratories, including the Russians.

France has now joined the US and Germany in a statement of solidarity with the UK that says there is no other plausible explanation than Russian responsibility. But the initial French reaction was to insist on evidence first:

http://www.euronews.com/2018/03/15/france-agrees-with-uk-that-russia-to-blame-for-spy-poison-attack

If, as is plausible, somebody with access to illegal Russian stockpiles did this without authority one would expect the Russian government to try to evade responsibility while being extremely cooperative and anxious to help establish the facts so as to ensure any culprits were found and stopped from seriously damaging Russian as well as other interests.

Instead Russia is churning out the usual stuff from Sputnik and threatening retaliation.

If that is the result of ineptitude on the part of both the British and Russian governments that is not terribly unusual.

But there is the other alternative that the Russians were deliberately testing Western responses. So far those responses are completely inadequate if based on a firm conclusion that the Russian government is either complicit in testing Western responses or unable to control its security personnel who decide to do so without authority.