The unexplained death of a researcher at a Russian anti-plague institute has one former senior U.S. arms official warning it could be 'far worse' than the bacteria that caused the Black Death.

Marshall Billingslea, who served as President Donald Trump's special presidential envoy for arms control, said a top Soviet bioweapons defector told him decades ago about secret work on antibiotic-resistant plague at the same facility where the recent plague-related death occurred.

"This may not be just 'the plague.'

It may be far worse," Billingslea wrote on He spent much of the 1990s working on proliferation and arms control issues on the Senate Foreign Relations Committee — and during that time worked with Ken Alibek, the former No. 2 official in the Soviet biological weapons program who defected to the U.S.

BIOWEAPON THREAT EXPOSED AS ANTHROPIC SOUNDS ALARM ON FOREIGN ACTORS PLOTTING VIRUS EXPERIMENTS "He claimed that they had engineered a strain of plague that was antibiotic-resistant & that they had worked on plague chimeras (fusing 2 or more pathogens)," Billingslea wrote of Alibek.

"He specifically called out Irkutsk for being involved in the BW program," he added.

Billingslea's warning comes after a 28-year-old employee of the Irkutsk Anti-Plague Institute of Siberia and the Far East died last week from what Russian authorities have described as pneumonia of undetermined origin.

Russia has not said the woman died of plague and said testing found no microorganisms in her body linked to her work.

The World Health Organization (WHO) said Tuesday that Russian officials told the agency no plague or other high-threat pathogen had been detected among her close contacts, though they are still awaiting confirmation of what caused her fatal pneumonia.

DEFENSE DEPARTMENT SCIENTIST’S ACCIDENTAL DEATH RAISES QUESTIONS AS PROBE INTO MISSING SCIENTISTS GROWS About 60% of the woman's identified contacts have been tested so far, according to Russian authorities, with two cases of COVID-19 and two rhinovirus infections detected — but no other infectious disease pathogens.

Still, the unexplained death at a facility that works with dangerous pathogens has drawn scrutiny from biosecurity experts — including questions about why the cause remains unknown. Kevin Esvelt, an MIT professor who studies biological security and catastrophic pandemic risks, said Russia's latest explanation suggests the researcher did not die from plague while leaving unanswered what killed her. "If we take the Russian authorities at their word, it is not plague," Esvelt wrote Tuesday. "But untargeted research-grade sequencing reliably identifies pathogens, and people seldom die of pneumonia without pathogens, let alone 28-year-olds," he added.

"It would be surprising if they did not know the cause."

Esvelt cautioned that more alarming explanations remain hypothetical.

"The concerning possibility is still only a possibility," he wrote. The questions surrounding the death come against the backdrop of the Soviet Union's documented history of secretly developing biological weapons, including weaponized plague, and U.S. assessments that elements of that program continued after the Soviet collapse.

In its 2025 arms control compliance report, the State Department assessed that Russia maintains an offensive biological warfare program in violation of the Biological Weapons Convention and that the Soviet program was "absorbed, not dismantled" after the Soviet collapse.

The department said the program "has continued and evolved" under the Russian Federation and that Moscow continues activities prohibited under the convention. Russia has repeatedly denied maintaining an offensive biological weapons program. The U.S. assessment does not establish any connection between Russia's biological weapons program and the researcher's death in Irkutsk, and no such connection has been confirmed. NEW YORK DECLARES MEASLES DISASTER EMERGENCY AS CASES SURGE IN PENNSYLVANIA The Soviet program Billingslea referenced was one of the world's largest clandestine biological weapons efforts. Alibek, born Kanatjan Alibekov, rose to become the first deputy chief of Biopreparat, the ostensibly civilian network that formed a major part of the Soviet biological weapons apparatus, before defecting to the United States in 1992.

After his defection, Alibek provided U.S. officials with extensive information about the scale and capabilities of the Soviet program. He described Soviet scientists working to weaponize pathogens including plague and developing strains designed to withstand antibiotics.

His claims about some aspects of the Soviet program have faced scrutiny over the years, making independent documentation particularly important. But the existence and enormous scale of the Soviet biological weapons effort — conducted despite Moscow's 1975 ratification of the Biological Weapons Convention — is well established.

The Irkutsk institute says it was founded in 1934 as a scientific and operational center for combating plague. Its mission later expanded to other highly dangerous diseases, including cholera and anthrax, and it describes itself today as a research and anti-epidemic facility responsible for helping protect the region from infectious disease.

The Trump administration has said it is closely monitoring the situation and pressed Moscow for greater transparency about what happened.

Secretary of State Marco Rubio said Monday that while travel between the U.S. and Russia remains limited, an infectious disease could quickly spread beyond Russia's borders. "If something like that gets out, it could quickly spread," Rubio said. President Donald Trump said the U.S. would offer assistance to Russia if needed.

"I hate to see them having it," Trump told reporters Monday. "It's a rough disease."