The laboratory, in a squat, beige building on the outskirts of Moscow, has an unremarkable name: Scientific Research Institute No 2, or NII-2 for short. Most evenings, a few lights are visible through the windows, framed by a couple of scrawny trees.

The lab’s precise function is a state secret. But numerous former Russian intelligence officers – some retired and some defectors – have confirmed that the building is home to the Kremlin’s infamous poisons factory, established in 1921 on the orders of Vladimir Lenin.

Western intelligence experts believe its efforts were originally directed at using poisons en masse on the battlefield. The KGB concluded that the substances worked better on individuals.

It is too early to say whether the nerve agent used against Sergei Skripal and his daughter, Yulia, in Salisbury last Sunday originated in Moscow. Scientists at a UK government facility in nearby Porton Down are carrying out tests to determine chemical “attribution”. There will be a footprint.

If the agent was a rare, bespoke toxin, this would be highly suggestive of state involvement, one former Foreign Office official said. The official pointed out that the Soviet Union and Russia had a history of using poisons against alleged traitors, most famously killing Alexander Litvinenko in 2006 with a radioactive cup of green tea.

The most comprehensive account of what was referred to in Soviet official documents as Lab X comes from Pavel Sudoplatov, Stalin’s former spy chief. Sudaplatov wrote about the lab and its director, Prof Grigory Mairanovsky, in his 1994 memoir Special Tasks. It is a body-strewn account.

According to Sudaplatov, Maironovsky would inject people with poison under the guise of a routine medical checkup. The victims included Raoul Wallenberg, a Swedish diplomat who died mysteriously in Soviet custody, as well as Ukrainian nationalists and potential defectors. Soviet leaders gave the orders for execution. Sudaplatov covered up the operation afterwards.

The KGB was still silencing enemies during the late Soviet period. Oleg Kalugin, a KGB general, admitted that Lab X supplied the poison used to kill Georgi Markov, a Bulgarian dissident murdered in London in 1978 with a ricin-tipped umbrella. The Bulgarians carried out the operation. It was justified on the grounds that Markov was allegedly a British spy.

Yuri Shvets, a KGB colonel who later settled in the US, described a visit to the top-secret lab in the 1980s. He had come to pick up a truth-telling drug to use on an American source. The technical operations directorate of the KGB had to approve any use. Shvets wrote that the lab manufactured a wide variety of substances, including poisons, narcotics and psychotropic substances.

Shvets left with a vial of SP-117 – concentrated alcohol to be dropped into a champagne glass. He noted that if the drug – used to make a subject rapidly drunk – was number 117 then then KGB’s toxic arsenal probably contained at least another 116 potions. He nicknamed the “small, portly” lab worker who briefed him on the chemical effects Aesculapius, after the Greek god of medicine.

The lab fascinated Soviet leaders, including the last one, Mikhail Gorbachev. In 1988 Gorbachev asked for a briefing. The KGB refused.

Details of its secret operations leaked out after the breakup of the USSR. There is no up-to-date information about the lab’s current work. Intelligence sources think its function is largely similar to that in KGB times.

In the Litvinenko operation, polonium was produced at another state-controlled facility, in in the town of Sarov, a UK public inquiry was told in 2015. It is believed the Moscow lab converted the isotope into a form where it could be taken safely to London. The polonium dropped into Litvinenko’s teapot may have been carried in gelatin-coated micro-pellets.

When the FSB’s current “scientific institute” was built on an impregnable and isolated site, locals assumed it was to treat wounded soldiers from the Soviet war in Afghanistan. That wasn’t the case. Putin-era possible victims of its activities include Russian investigative journalists, Chechen rebels and Ukraine’s former pro-western president Viktor Yushchenko, who was poisoned with something like dioxin.

Sudaplatov’s telltale book would be impossible to write or publish in today’s Russia, where the price for spilling secrets is known. The ageing spy boss was convinced that no special operation could remain secret forever. If the attempted murder of Skripal was indeed a state plot, at some point, possibly decades from now, the full details are likely to emerge.

“This is one of the great lessons of the breakdown of the Soviet Union and Communist party rule,” Sudaplatov wrote. “Once the dam is broken, the flood of secret information is uncontrollable.”

