The head of Britain's military research center said on Tuesday it was unable yet to say whether the military-grade nerve agent that poisoned a Russian double-agent last month had been produced in Russia.
"We were able to identify it as Novichok, to identify that it was military-grade nerve agent," Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory at Porton Down in England, told Sky News.
"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."
However, he confirmed the substance required "extremely sophisticated methods to create, something only in the capabilities of a state actor".
He added: "we are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that."
Moscow has denied being behind the attack on Sergei Skripal and his daughter in the southern English city of Salisbury on March 4.