An explosive U.K. intelligence dossier made public Friday claims Russia tested the use of door handles as a way to infect people with nerve agents before an ex-spy and his daughter were poisoned exactly that way last month.

In a letter to NATO, the United Kingdom’s national security chief, Sir Mark Sedwill, said the Kremlin set up a program in the 2000s under the code name Foliant that tested the means for delivering chemical warfare agents and to “train personnel from special units in the use of these weapons.”

“This program subsequently included investigation of ways of delivering nerve agents, including by application to door handles,” he wrote, according to the Guardian. “Within the last decade, Russia has produced and stockpiled small quantities of novichok under the same program.”

DAUGHTER OF POISONED SPY SERGEI SKRIPAL TURNS DOWN RUSSIAN HELP

U.K. officials have said Sergei Skripal, 66, and his 33-year-old daughter, Yulia, were poisoned when they came in contact with a chemical nerve agent at his home in Salisbury on March 4. Samples taken from the home found a high concentration of the nerve agent – believed to be Soviet-produced novichock – on the door handles.

The letter, which was directed to NATO Secretary General Jens Stoltenbeg, emphasized England’s belief Russia is responsible for poisoning the Skripals. Russia has denied all involvement.

Additionally, Sedwill wrote evidence was discovered showing Russian military intelligence had targeted the email accounts of both victims since at least 2013.

He said Russia regarded at least some of its defectors as “legitimate targets for assassination,” with the suggestion Sergei Skripal -- a former member of Russian military intelligence sent to prison in 2004 after working with the U.K. -- was on the list.

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“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.

At a press conference arranged before the letter’s release, Russian ambassador to the U.K. Alexander Yakovenko said he was not familiar with the latest allegations.

“If somebody was spying, why were the British services not complaining about that,” he said, according to Sky News. “We didn’t hear any signs, any applications from the British side.”

The letter was made public a day after investigators at the Organization for the Prohibition of Chemical Weapons confirmed British findings that the Skripals were poisoned with a military-grade nerve agent. The watchdog did not say who was responsible for the poisoning.

On Friday, Russian Foreign Minister Sergey Lavrov said the report does nothing to back the British allegations that Moscow was behind the attack, which Moscow denies.

"I want to underline: the OPCW only has confirmed the composition of the chemical agent," Lavrov said at a news conference.

Yulia Skripal, 33, has been released from the hospital. Her father remains in the hospital but British health officials say he is improving.

Russia has charged that British officials were keeping Skripal's daughter in isolation. It demanded access to her and prodded Britain to share evidence in the case.

The Associated Press contributed to this report.