Despite being 95 years old, Hideo Shimizu still remembers the 4½ months he spent as a teenager with Unit 731, a biological and chemical warfare unit of the now-defunct Imperial Japanese Army, as if they were just yesterday.

It was only three days after Shimizu had graduated from grade school in March 1945 that he was recruited into the military at the age of 14 as a “technical trainee” and sent to the unit’s headquarters in the Pingfang district of Harbin, a key city in the Japanese puppet state of Manchukuo in present-day northeastern China.

He had no idea what was in store for him at the unit when he was assigned to the “education department” at its headquarters, a three-story modern building equipped with labs and prisons.