The Siberian complex known as Vector was a top Soviet research facility for bioweapons. Today, its scientists study defenses against Ebola and other pathogens. A lab worker accidentally contracted Ebola in 2004 while working on vaccines. (Joby Warrick/The Washington Post)

She was an ordinary lab technician with an uncommonly dangerous assignment: drawing blood from Ebola-infected animals in a secret military laboratory. When she cut herself at work one day, she decided to keep quiet, fearing she’d be in trouble. Then the illness struck.

“By the time she turned to a doctor for help, it was too late,” one of her overseers, a former bio­weapons scientist, said of the accident years afterward. The woman died quickly and was buried, according to one account, in a “sack filled with calcium hypochlorite,” or powdered bleach.

The 1996 incident might have been forgotten except for the pathogen involved — a highly lethal strain of Ebola virus — and where the incident occurred: inside a restricted Russian military lab that was once part of the Soviet Union’s biological weapons program. Years ago, the same facility in the Moscow suburb of Sergiev Posad cultivated microbes for use as tools of war. Today, much of what goes on in the lab remains unknown.

The fatal lab accident and a similar one in 2004 offer a rare glimpse into a 35-year history of Soviet and Russian interest in the Ebola virus. The research began amid intense secrecy with an ambitious effort to assess Ebola’s potential as a biological weapon, and it later included attempts to manipulate the virus’s genetic coding, U.S. officials and researchers say. Those efforts ultimately failed as Soviet scientists stumbled against natural barriers that make Ebola poorly suited for bio­warfare.

The bioweapons program officially ended in 1991, but Ebola research continued in Defense Ministry laboratories, where it remains largely invisible despite years of appeals by U.S. officials to allow greater transparency. Now, at a time when the world is grappling with an unprecedented Ebola crisis, the wall of secrecy surrounding the labs looms still larger, arms-control experts say, feeding conspiracy theories and raising suspicions.

“The bottom line is, we don’t know what they’re doing with any of the pathogens in their possession,” said Amy Smithson, a biological weapons expert who has traveled to several of the labs and written extensively about the Soviet-era weapons complex.

At least four military labs have remained off-limits to any outside scrutiny since the end of the Cold War, even as civilian-run institutions adopted more transparent policies and permitted collaborations with foreign researchers and investors, U.S. officials and weapons experts say. Even acknowledging — as most experts do — that Russia halted work on offensive bio­weapons decades ago, the program’s opacity is a recurring irritant in diplomatic relations and a source of worry for security and health experts who cite risks ranging from unauthorized or rogue experiments to the theft or accidental escape of deadly microbes.

Enhancing the threat is the facilities’ collection of deadly germs, which presumably includes the strains Soviet scientists tried to manipulate to make them hardier, deadlier and more difficult to detect, said Smithson, now a senior fellow with the James Martin Center for Nonproliferation Studies, a research institute based in Monterey, Calif.

“We have ample accounts from defectors that these are not just strains from nature, but strains that have been deliberately enhanced,” she said.

Other countries, including the United States, also conduct military research on defending against biological threats, including Ebola — a fact that draws criticism from some health experts and charges of hypocrisy from Russia. Pentagon officials counter that U.S. bio­defense laboratories are subject to oversight and regular inspections by outside agencies.

Russian officials defend their right to military secrecy and point to tangible benefits from years of Ebola research. This month, Russian officials announced experimental Ebola vaccines developed by the same two labs that lost workers to Ebola accidents: the Defense Ministry’s Microbiology Research Institute at Sergiev Posad and the Vector Center for Virology and Biotechnologies in the Siberian city of Novosibirsk.

“Vaccines are ready,” Valery Chereshnev, chairman of a science committee in the Russian parliament, told the news agency Tass last week.

An unidentified scientist wears a protective suit in June 2002. The suit was used while working with Vector’s viral collection, which currently includes most of the world’s most hazarous pathogens. (Joby Warrick/The Washington Post)

Two Ebola accidents

The Sergiev Posad lab was the site of the first of the two Ebola accidents, which today remain the only known cases in which lab workers died from inadvertent exposure to the virus. Similar exposures occurred in labs in the United States, Germany and Britain, but in those cases the victims survived.

In the 1996 incident, first documented in Russian-language news accounts and later described by author David Quammen in his 2012 book, “Spillover,” a worker named Nadezhda Makovetskaya cut herself at a facility that was developing an experimental treatment for Ebola derived from blood serum from horses.

The job was unusually risky because of the test animals’ size, Lt. Gen. Valentin Yevstigneyev, a Defense Ministry official overseeing bio­defense work, was quoted as telling Russian journalists.

“It is difficult to describe working with a horse infected with Ebola,” he said. “One false step, one torn glove and the consequences would be grave.”

Despite wearing layers of protective clothing, the woman suffered a cut that penetrated her gloves, he said. Makovetskaya hid the accident from her bosses­ until it was too late, he said. Her death would be noted in records of the World Health Organization as the first Ebola fatality stemming from a laboratory accident anywhere in the world.

In the second incident, a Russian lab worker contracted Ebola in 2004 while working with infected guinea pigs in the Vector virology research center outside Novosibirsk. The victim, Antonina Presnyakova, 46, was drawing blood from one of the animals on May 4 when she accidentally pricked her left hand with a needle that pierced two layers of gloves.

Presnyakova was immediately hospitalized, but despite medical treatment she contracted the disease and died two weeks later.

A notorious past

The facilities that reported the accidents have a notorious past, having once been part of a larger complex of Soviet laboratories and testing facilities devoted to the science of biological warfare.

Russian President Boris Yeltsin confirmed the existence of the secret program to top U.S. officials in the early 1990s after declaring an end to bio­weapons research in the months after the Soviet Union’s dismantling. Afterward, successive U.S. administrations dispatched experts and resources to the former Soviet republics to help secure dangerous pathogens and support the transition to peaceful research at civilian-run labs, including Vector, one of two known repositories for the smallpox virus.

U.S. experts collected first-person accounts of the research and visited outdoor testing facilities where dogs, monkeys and other animals were exposed to deadly pathogens, encounters described in the Pulitzer Prize-winning history “The Dead Hand,” by former Washington Post editor David Hoffman. But Russian officials refused to grant access to military laboratories and never offered a full accounting of past weapons research or described how they disposed of weaponized biological agents.

But more recently, new historical scholarship, drawing from Soviet-era records and interviews with Russian scientists, has offered deeper insight into Soviet efforts to make weapons out of a wide range of pathogens, from anthrax bacteria to the viruses that cause Marburg fever and Ebola.

According to these accounts, much of the Ebola research appears to have been devoted to developing vaccines to protect Red Army troops against the disease. But scientists also ran experiments intended to optimize the virus’s growth and isolate the parts of its genome that make it deadly, said Raymond Zilinskas, a microbiologist and co-author of “The Soviet Biological Weapons Program,” an exhaustive history published in 2012 by Harvard University Press.

As Soviet scientists worked in secret to manipulate the virus, other teams constructed large fermenters and production facilities that could reproduce the altered pathogens on an industrial scale, Zilinskas said.

“There is only one reason why you would have a large production of these viruses, and that’s for offensive purposes,”said Zilinskas, who, along with co-author Milton Leitenberg, spent more than a decade interviewing Russian scientists and other officials with direct knowledge of the program.

In the years just before the Soviet Union’s collapse, the program’s managers plunged into novel experiments — with code names such as “Hunter” and “Bonfire” — that sought to create super­bugs that would resist common antibiotics, or combine elements of different microbes to increase their lethality. Sergei Popov, a former Vector scientist who defected to the West, described work on creating a “completely artificial agent with new symptoms, probably with no ways to treat it.”

“Nobody would recognize it. Nobody would know how to deal with it,” Popov said in an interview broadcast on the PBS program “Nova” in 2002, a few years after the scientist settled in the United States. Popov declined a request for an interview this week.

Ultimately, the effort to concoct a more dangerous form of Ebola appears to have failed. Mutated strains died quickly, and Soviet researchers eventually reached a conclusion shared by many U.S. bio­defense experts today: Ebola is a poor candidate for either biological warfare or terrorism, compared with viruses such as smallpox, which is highly infectious, or the hardy, easily dispersible bacteria that causes anthrax.

Things might have turned out differently if the Soviets had continued their work, Zilinskas and Leitenberg suggest in their account. The science of genetic modification was still in its infancy at the time Yeltsin outlawed the program, essentially freezing the research in place.

“Most, if not all, of the recombinants created in the laboratory were not close to being weaponized,” the book states. Still, it adds: “One must assume that whatever genetically engineered bacterial and viral forms were created . . . remain stored in the culture collections of the Russian Federation Ministry of Defense.”