Soviet officials privately admitted to building and stockpiling biological weapons while publicly denying their existence, internal documents reveal.

That history is back in focus as biosecurity experts question Russia’s reported response to the unexplained death of a worker at a plague research lab.

Darya Shipilova, 28, a worker at the Irkutsk Anti-Plague Research Institute in Siberia, died on October 2 from what Russian authorities describe as pneumonia of unknown origin.

Nearly 200 contacts were placed under medical observation and tested, although Russian authorities told the World Health Organization on Tuesday that no case of plague had been recorded in Irkutsk.

‘The Soviet Union had an offensive biological weapons program … and some of those anti-plague institutes were part of that program,’ Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told Newsweek.

A May 1990 report sent to Soviet leader Mikhail Gorbachev, published by the National Security Archive in 2010, acknowledged that biological weapons had been developed, produced and stored. 

It said Soviet organizations had developed 12 formulations and methods of using them by 1985. 

Four months earlier, Soviet Defense Minister Dmitry Yazov had urged officials to withdraw a proposed disclosure about past stockpiles because it would contradict Moscow’s public denials, according to a separate memo.

While Russian officials have not confirmed plague as the cause of Shipilova's death, the lab worker reportedly told medical staff in late September that she had accidentally broken a test tube holding live bacteria, according to the news outlet Lyudi Baikala. 

Through an international health alert system, Russian health officials on Tuesday said a full range of preventive measures had been carried out, including tracing of the deceased woman’s contacts, medical monitoring and lab tests. 

Russia’s reported containment measures have prompted some experts to question whether officials were investigating a pathogen other than plague that they have not publicly identified. 

'Just because there's plague on the door doesn't mean that's the only thing they work on. They work on many different biological agents there,' Adalja told Newsweek.

However, no evidence links the incident to a biological weapon. 

In the report sent to Gorbachev, Lev Zaikov, the Politburo member overseeing the Soviet military-industrial complex, stated that the Soviet Union began developing biological weapons in the 1950s at three USSR Defense Ministry organizations. 

That group was later joined by another 12 organizations in 1971. 

'By 1985, they had developed 12 recipes and means for using them. These were produced in suitable quantities, stored, and destroyed after the expiration of useful life (an average of 6 months),' Zaikov wrote.

The Soviet Union had signed the Biological Weapons Convention in 1972 alongside the US, the UK and dozens of other countries. 

The treaty entered into force in 1975, prohibiting the development, production, stockpiling, acquisition and transfer of biological weapons. 

Zaikov argued that the treaty lacked effective inspections and a clear distinction between weapons development and defensive research.

He alleged that NATO countries exploited those gaps to develop and produce highly virulent infectious agents, with some work conducted in countries that had not signed the treaty. The document portrayed Soviet biological weapons work, which it said continued until 1989, as a response to those alleged activities.

Citing progress in chemical weapons negotiations and the prospect of international inspections, the report said an October 6, 1989, Central Committee decree ordered Soviet biological weapons research to shift toward defensive work to comply with treaty obligations.

It also claimed that Soviet stockpiles had been destroyed in 1988, industrial production had stopped, and equipment used to process biological agents and assemble munitions had been dismantled.

The National Security Archive described Zaikov’s report as putting a ‘very selective spin’ on the program’s history and said the document showed Soviet officials had also misled Gorbachev.

Vladimir Pasechnik, who headed a scientific organization in Leningrad, defected to Britain in October 1989 and began disclosing details of the Soviet program.

Zaikov’s report acknowledged that Pasechnik knew about the secret biological research and the facilities involved.

It downplayed the technical damage his disclosures could cause but warned they could lead Western governments to question Moscow’s compliance with the Biological Weapons Convention.

In a January 10, 1990 memo, Soviet Defense Minister Dmitry Yazov opposed a Foreign Ministry proposal to confidentially share information with the US about biological weapons stockpiles and production sites that existed before the treaty took effect in 1975.

He warned that the disclosure would contradict Moscow’s official claim that it had never developed, produced or possessed biological weapons and could damage the country’s international standing.

Yazov urged officials to recall the draft and maintain the existing official position.

The memo also rejected a proposed acknowledgment of a laboratory accident during the 1979 anthrax outbreak in Sverdlovsk. 

Yazov insisted the outbreak was foodborne and urged officials to postpone any visit by American experts to the military facility until arrangements for monitoring treaty compliance had been worked out.

A 1994 scientific investigation later concluded that the Sverdlovsk outbreak was caused by airborne anthrax released from the military facility, contradicting the Soviet explanation that contaminated food was responsible.