Question: Unit Ei 1644  was established in 1939 in Japanese-occupied Nanking as a satellite unit of Unit 731,  a covert biological and chemical warfare research and development unit of  what army?
Sentence (from "American cover-up of Japanese war crimes"): The American cover-up of Japanese war crimes occurred after the end of World War II, when the occupying US government granted political immunity to military personnel who had engaged in human experimentation and other crimes against humanity, predominantly in mainland China.
Relevant evidence: No