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 "Unit Ei 1644"): Unit Ei 1644 (Japanese: 栄1644部隊 ) — also known as Unit 1644 — was a laboratory and one of the water purification centers established under control of the Epidemic Prevention and Water Purification Department.
Relevant evidence: Yes