The release of WWII-era military documents this year has given a boost to researchers digging into Japan’s notorious germ warfare program, which lasted from 1936 to 1945. As Japan’s defeat loomed, unit 731 members were instructed to destroy evidence — and witnesses. Unit 731 doctors called the people they experimented on “maruta,” or logs, in other words, not human. Unit 731 is estimated to have killed around 3,000 people, while bioweapons developed by other branches of the program are believed to have killed far more. The U.S. government kept the details of Unit 731 and its immunity deal secret for decades.