A program set up in Russian Federation in the 2000s trained personnel from special units in the use of chemical warfare agents, he said, including investigating how nerve agents could be administered through door handles. "There is no plausible alternative explanation".
The Russian Embassy has sensationally claimed poisoned former spy Sergei Skripal and his daughter Yulia are being held prisoner by the UK.
The latest such case is the death of Russian businessman Nikolay Glushkov on March 12, after the apparent attack on the Skripals, Yakovenko said.
"It is highly likely that the Russian intelligence services view at least some of its defectors as legitimate targets for assassinations", Sedwill writes, alluding to a possible motive for the attack on Sergei Skripal, a former Russian military official convicted of being a British spy.
In a letter to North Atlantic Treaty Organisation secretary general Jens Stoltenberg on Friday, Sedwill said: "We therefore continue to judge that only Russia has the technical means, operational experience and motive for the attack on the Skripals and that it is highly likely that the Russian state was responsible".
Yulia was discharged from the hospital earlier this week and is being kept in an unknown location.
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Sedwill said "credible open-source reporting and intelligence" showed that in the 1980s the Soviet Union developed a family of nerve agents known as Novichoks at a base in Shikhany near Volgograd.
Sedwill goes on to claim that Russian Federation was likely the only former Soviet republic to pursue "an offensive chemical weapons programme after independence".
Richard Guthrie, an independent chemical-weapons expert, says an important detail in the investigation is that the toxic substance is of "high purity".
Mr Sedwill's letter provided some detail of the intelligence Prime Minister Theresa May referred to in the House of Commons when she claimed Russian Federation was "highly likely" to have been behind the poisoning. And Sedwill added that since 2006, there "have been numerous suspected Russian state-sponsored assassinations outside the former Soviet Union". Alexander Yakovenko also accused the British authorities of destroying evidence in the Salisbury attack and said the British government had yet to produce the evidence to back its claims that Russian Federation was responsible.
He said, based on British intelligence, that Russian Federation had developed a next generation nerve-agent group called Novichok and stockpiled small amounts of the agent within the last decade.
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