In a letter made public Friday, British National Security Adviser Mark Sedwill said that Russian intelligence agencies have been spying on Sergei and Yulia Skripal for at least five years.
Mark Sedwill also said in a letter to NATO Secretary General Jens Stoltenberg that Russian Federation had previously tested whether door handles could be used to deliver nerve agents.
"We have information indicating Russian intelligence service interest in the Skripals, dating back at least as far as 2013, when e-mail accounts belong to Yulia were targeted by GRU cyber specialists", Sedwill wrote.
By 1993, when Russia signed the Chemical Weapons Convention, Sedwill said it was "likely" that some Novichoks had passed testing to allow their use by the Russian military.
On Thursday, OPCW confirmed Britain's finding that Skripal and his daughter were poisoned with a nerve agent in the English city of Salisbury on March 4.
Richard Guthrie, an independent chemical-weapons expert, says an important detail in the investigation is that the toxic substance is of "high purity".
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On March 4, Sergei Skripal, who had been convicted in Russian Federation of spying for Great Britain, and his daughter Yulia suffered the effects of an alleged nerve agent in the British city of Salisbury.
In his letter, Sedwill outlines how the Soviet Union "developed a new class of "fourth generation" nerve agents, known as Novichoks" during the 1980s and Russian Federation was probably the only former Soviet republic to pursue "an offensive chemical weapons programme after independence".
He then claimed Russian Federation saw at least some of its defectors as legitimate targets for assassination, suggesting Skripal could be among them.
Among the techniques they had investigated for delivering nerve agents was applying them to door handles, he said.
"If someone was spying, why were the British services not complaining about that?"
Former KGB bodyguards identified as suspects in the murder denied any involvement.
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Sir Mark said: "There is no plausible alternative explanation", adding that Russian Federation had continued to produce and stockpile small quantities of Novichok within the last decade.
Britain blamed Russian Federation for the poisoning and Prime Minister Theresa May said the Skripals had been attacked with a military-grade nerve agent from the Novichok group of poisons, developed by the Soviet Union.
"We didn't produce Novichok, we didn't store this Novichok, so-called under the Western classification, was never in our military forces".
In an account based in part on declassified United Kingdom intelligence, Sir Mark also said the Russians had developed a programme in the 2000s to train personnel from "special units" in the use of chemical warfare agents.
"We get the impression the British Government is deliberately pursuing the policy of destroying all possible evidence".
Sergei Skripal remains in hospital in serious condition, while Yulia Skripal has been released.
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