Marshall Billingslea was a Senate Foreign Relations Committee staffer in the 1990s when he sat down with one of the highest-ranking defectors from the Soviet biological weapons program and began learning the extent of what Moscow had secretly built.

And what he found out from Ken Alibek, the former No. 2 official in the Soviet biological weapons apparatus, is raising red flags for Billingslea over the Oct. 2, 2026, death linked to an anti-plague institute in Russia.

"I think one of the most important things that Ken shed light on was the sheer scale of the Soviet program, which then Russia inherited," Billingslea told Fox News Digital. "Just mammoth." Three decades later, the mysterious death of a researcher at the Irkutsk Research Anti-Plague Institute of Siberia and the Far East has put new attention on what became of that program — and Billingslea argues the world should be demanding answers from Moscow. MYSTERY DEATH AT RUSSIAN PLAGUE LAB TRIGGERS SWEEPING PRECAUTIONS AS US WATCHES CLOSELY "It's not acceptable in this day and age for the Russians to have a biological warfare program," said Billingslea, who previously served as President Donald Trump's special presidential envoy for arms control. "They need to cut it out, and they need to tell us. They need to come clean about what they're doing at this lab and whether this lab is still involved in their warfare program."

The U.S. government says Russia's offensive biological weapons program did not disappear with the Soviet Union.

"Russia maintains an offensive biological weapons program and is in violation of its obligations under the BWC," a State Department official told Fox News Digital this week.

BWC, or Biological Weapons Convention, is an international treaty that prohibits the development, production and stockpiling of biological weapons.

State's 2025 arms control compliance report assessed that the Soviet-era program was "absorbed, not dismantled" by Russia after the collapse and has "continued and evolved" since then.

There is no evidence linking the Irkutsk researcher's death to Russia's biological weapons program, and Billingslea told Fox News Digital he has seen no evidence that the the institute remains involved in the program today.

But the unexplained death has revived scrutiny of the Soviet program's work and the network of ostensibly civilian laboratories that Billingslea said Alibek identified as also performing classified work.

Alibek's disclosures included Soviet efforts to alter pathogens to make them more lethal, transmissible or difficult to treat, Billingslea said, adding that some of the most troubling revelations involved plague.

BIOWEAPON THREAT EXPOSED AS ANTHROPIC SOUNDS ALARM ON FOREIGN ACTORS PLOTTING VIRUS EXPERIMENTS "The goal was to render the plague bacteria far more resistant to antibiotics," he said.

Billingslea said Soviet researchers also experimented with combining plague with toxin-producing pathogens, with the aim of making infections even harder to treat. "They took the plague bacteria and they merged it with a non-dangerous bacteria," Billingslea said.

According to Alibek's account, researchers also worked on combining plague with toxin-producing pathogens so that treating an infected person with antibiotics could trigger the release of toxins.

Billingslea said that U.S. officials were not relying solely on Alibek for their understanding of the program and that there were "multiple other sources of information" that made him "very confident in the basics" regarding the disclosures regarding Soviet-era research.

"So this doesn't all derive from Ken Alibek," he insisted when speaking with Fox News Digital.

By the time Billingslea was interviewing Alibek in the mid-1990s, he said, Washington had already confronted Moscow over its biological weapons work. "We knew about the plague programs, we knew what they were up to, and we knew where the sites were," he said.

Among the facilities Alibek identified were at least six Soviet Ministry of Health laboratories that had legitimate civilian public-health missions while also performing classified work connected to the biological weapons program, according to Billingslea.

The Irkutsk institute was among the facilities Alibek identified, but Billingslea cautioned he does not recall the Soviet defector saying that antibiotic-resistant plague or plague chimeras were specifically developed there.

That distinction has become particularly important following the unexplained death of a 28-year-old researcher at the Irkutsk Anti-Plague Research Institute.

Russian authorities have said they found no microorganism linked to the woman's professional activities, while the World Health Organization has sought additional information from Moscow about what caused her fatal pneumonia.

Billingslea said the pressure for answers — and for Russia to end its offensive biological weapons program — should not come from Washington alone. "It's very, very important that this is not viewed as a U.S. versus Russia discussion," he said. "This is a topic of grave concern to the entire international community." He also said that Russia needs to release the genetic sequence of the pathogen that killed the researcher, along with information about contact testing, the timeline of her illness and the institute's biosafety procedures.

The genetic sequence could allow outside experts to determine "what it was and whether it was modified," he said.

Billingslea called on countries outside the traditional U.S. alliance system, including Brazil and Mexico, to press Moscow for greater transparency, and pointed to a December meeting of parties to the Biological Weapons Convention as an opportunity for countries to confront Russia over the program. "At the December meeting, Russia is going to get grilled," he said. "And they should."