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?

Context paragraphs:
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.  The pardon of Japanese war criminals, among whom were Unit 731's commanding officers General Shiro Ishii and General Masaji Kitano, was overseen by General of the Army Douglas MacArthur in September 1945.  While a series of war tribunals and trials was organized, many of the high-ranking officials and doctors who devised and respectively performed the experiments were pardoned and never brought to justice.  As many as 12,000 people, most of them Chinese, died in Unit 731 alone and many more died in other facilities, such as Unit 100 and in field experiments throughout Manchuria.

Zhongma Fortress: Zhongma Fortress — or Zhong Ma Prison Camp, or Unit Tōgō — was a prison camp where the Japanese Kwantung Army carried out covert biological warfare research on human test subjects.  Built in Beiyinhe, outside of Harbin, Manchukuo during the Second Sino-Japanese War, the camp served as a center for human subject experimentation and could hold up to 1,000 prisoners at any given time.  In 1937 the prison camp was destroyed and testing operations were transferred to Pingfang under Unit 731.

Unit 516: Unit 516 (第五一六部隊) was a top secret Japanese chemical weapons facility, operated by the Kempeitai, in Qiqihar, Japanese-occupied northeast China.  The name Unit 516 was a code name (Tsūshōgō) of the Unit.  It was officially called the Kwantung Army Chemical Weapons Section and operated underneath Unit 731.

Biohazard (book): Biohazard, subtitled The Chilling True Story of the Largest Covert Biological Weapons Program in the World - Told from Inside by the Man Who Ran It, is the title of a 1999 book by former Soviet biological warfare researcher Ken Alibek that purports to expose the former Soviet Union's extensive covert biological weapons program.

267th Chemical Company: The 267th Chemical Company was a military unit of the U.S. Army Chemical Corps responsible for the surety of chemical warfare agents dubbed "RED HAT" deployed to the Islands of Okinawa, Japan and subsequently Johnston Atoll in the Pacific Ocean.  A recently discovered Army document reveals that the true mission of the 267th Chemical Company was the operation of the Okinawa deployment site as part Project 112.  Project 112 was a 1960s biological warfare field test program that was conducted by the Deseret Test Center.  Okinawa is not listed as a test site under Project 112 by the U.S. Department of Defense.

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.  It was established in 1939 in Japanese-occupied Nanking as a satellite unit of Unit 731.

Biological warfare: Biological warfare (BW)—also known as germ warfare—is the use of biological toxins or infectious agents such as bacteria, viruses, and fungi with the intent to kill or incapacitate humans, animals or plants as an act of war.  Biological weapons (often termed "bio-weapons", "biological threat agents", or "bio-agents") are living organisms or replicating entities (viruses, which are not universally considered "alive") that reproduce or replicate within their host victims.  Entomological (insect) warfare is also considered a type of biological weapon.  This type of warfare is distinct from nuclear warfare and chemical warfare, which together with biological warfare make up NBC, the military acronym for nuclear, biological, and chemical warfare using weapons of mass destruction (WMDs).  None of these are conventional weapons, which are deployed primarily for their explosive, kinetic, or incendiary potential.

Masaji Kitano: Masaji Kitano (北野政次 July 14, 1894 – May 17, 1986) was a medical doctor, microbiologist and the lieutenant general of the Imperial Japanese Army.  He was the second commander of Unit 731, a covert biological and chemical warfare research and development unit responsible for some of the most notorious war crimes carried out by Japanese personnel.

Unit 8604: Unit 8604 () was the Epidemic Prevention and Water Purification Department unit of the Japanese Southern China Area Army.  It allegedly secretly researched biological warfare and other topics through human experimentation during the Second Sino-Japanese War (1937–1945) and World War II era.  It was formed in Canton (Guangzhou), a city in southern China, and headquartered at Sun Yat-sen University of Medical Sciences (Hal Gold, "Unit 731 Testimony", 2003, p. 50).

Unit 731: Unit 731 (Japanese: 731部隊 , Hepburn: Nana-san-ichi Butai ) was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army that undertook lethal human experimentation during the Second Sino-Japanese War (1937–1945) of World War II.  It was responsible for some of the most notorious war crimes carried out by Japan.  Unit 731 was based at the Pingfang district of Harbin, the largest city in the Japanese puppet state of Manchukuo (now Northeast China).

Supporting paragraphs: Unit Ei 1644 Unit 731
Answer: the Imperial Japanese Army