Porton Down is UK government military research lab.
4/4/18, "Porton Down experts unable to identify 'precise source' of novichok that poisoned spy," Sky News, by Paul Kelso, Health Correspondent
"Scientists from Porton Down have not been able to establish where the novichok nerve agent used to poison Sergei and Yulia Skripal was made.
He said: "We were able to identify it as novichok, to identify that it was military-grade nerve agent.
"We have not identified the precise source, but we have provided the scientific info to Government who have then used a number of other sources to piece together the conclusions you have come to."
He said establishing its origin required "other inputs", some of them intelligence-based, that the Government has access to.
Mr Aitkenhead added: "It is our job to provide the scientific evidence of what this particular nerve agent is, we identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured."
However, he confirmed the substance required "extremely sophisticated methods to create, probably something only in the capabilities of a state actor".
He said there was no known antidote to novichok.
Porton Down's boss would not comment on whether the lab had developed or keeps stocks of novichok, but dismissed suggestions the substance used to poison the Skripals had come from Porton Down.
"There is no way anything like that could have come from us or left the four walls of our facility," said Mr Aitkenhead....
The Defence Science and Technology Laboratory also responded to Mr Aitkenhead's comments, writing in a series of posts on Twitter:
"Our experts have precisely identified the nerve agent as a novichok.
"It is not, and has never been, our responsibility to confirm the source of the agent.""...