“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.