Not everyone believed the explanation. Anthrax spores do naturally live in the soil and occasionally sicken people—but never before on this scale. The U.S. intelligence community had already gathered information, classified of course, pointing to an accident at a bioweapons lab. “I knew they were lying,” says Philip Russell, a now-retired Army infectious disease researcher, “and I knew they were lying because I had been briefed on Defense Intelligence Agency evidence.” For the public and for academic scientists, irrefutable evidence of a bioweapons facility at Sverdlovsk didn’t come until 1992, after the fall of the Soviet Union.

That year, Harvard biologist Matthew Meselson led a team of American scientists to Russia to investigate. Meselson was the one who urged the Soviet officials to give the National Academy of Sciences presentation—and the team was inclined to believe the Soviet explanation. But in Sverdlovsk, now renamed Yekaterinburg, they found evidence that flat-out contradicted the official line.

That evidence was the bodies of the victims. The pathologists who performed the autopsies back in 1979, Faina Abramova and Lev Grinberg had hidden the tissue samples, preserved all this time with formaldehyde and embedded in paraffin wax. In her book Anthrax: The Investigation of a Deadly Outbreak, Jeanne Guillemin, Meselson’s wife and a member of the U.S. team, describes how Abramova saved the autopsy samples:

In 1979, she took responsibility for hiding her materials when she knew the KGB wanted the autopsy records. She has told us she placed the jars of gross organs preserved in formaldehyde in the hospital’s pathology museum, on shelves among other such jars, like so many purloined letters. She hid the tissue samples in an innocuous corner cabinet near the autopsy room. When I asked her why she did this, she replied proudly, “It was my work!”

When David Walker, a pathologist at the University of Texas Medical Branch and another member of the team, got a look at the photos from the Soviet pathologists, he knew immediately. “From the very first picture they showed me,” he says, “I knew it was inhalation anthrax,” says Walker. Anthrax spores, when inhaled, germinate in lymph nodes in the chest, causing them to swell with blood and fluid. That’s exactly what Walker saw. The victims clearly did not die of intestinal anthrax from eating contaminated meat; they had died of inhalation anthrax, after breathing in spores.

Other evidence piled up. The U.S. team interviewed family members of the patients, and a map of their locations showed them all downwind of the bioweapons facility. And in 1992, Ken Alibeck, a Soviet scientist who worked on the bioweapons program, defected to the U.S. He would go on to co-write Biohazard: The Chilling True Story of the Largest Covert Biological Weapons Program in the World—Told from Inside by the Man Who Ran It, which said the anthrax got out when an air filter at the Sverdlovsk plant was not properly replaced.