https://en.wikipedia.org/wiki/Unit_731#Surrender_and_immunity

Destruction of evidence

As the Second World War started to come to an end, all prisoners within the compound were killed to conceal evidence, and there were no documented survivors.[102] With the coming of the Red Army in August 1945, the unit had to abandon their work in haste.

Curious.

American grant of immunity

Interesting.

Among the individuals in Japan after its 1945 surrender was Lieutenant Colonel Murray Sanders, who arrived in Yokohama via the American ship Sturgess in September 1945. Sanders was a highly regarded microbiologist and a member of America's military center for biological weapons. Sanders' duty was to investigate Japanese biological warfare activity. At the time of his arrival in Japan, he had no knowledge of what Unit 731 was.[69] Until Sanders finally threatened the Japanese with bringing the Soviets into the picture, little information about biological warfare was being shared with the Americans. The Japanese wanted to avoid prosecution under the Soviet legal system, so, the morning after he made his threat, Sanders received a manuscript describing Japan's involvement in biological warfare.[104] Sanders took this information to General Douglas MacArthur, who was the Supreme Commander of the Allied Powers and responsible for rebuilding Japan during the Allied occupations. MacArthur struck a deal with Japanese informants:[105] he secretly granted immunity to the physicians of Unit 731, including their leader, in exchange for providing America solely, with their research on biological warfare and data from human experimentation.[6] American occupation authorities monitored the activities of former unit members, including reading and censoring their mail.[106] The Americans believed that the research data was valuable and did not want other nations, particularly the Soviet Union, to acquire data on biological weapons.[107]

The Tokyo War Crimes Tribunal heard only one reference to Japanese experiments with "poisonous serums" on Chinese civilians. This took place in August 1946 and was instigated by David Sutton, assistant to the Chinese prosecutor. The Japanese defense counsel argued that the claim was vague and uncorroborated and it was dismissed by the tribunal president, Sir William Webb, for lack of evidence. The subject was not pursued further by Sutton, who was probably unaware of Unit 731's activities. His reference to it at the trial is believed to have been accidental. Later in 1981, one of the last surviving members of the Tokyo Tribunal, Judge Röling, had expressed bitterness in not being made aware of the suppression of evidence of Unit 731 and wrote, "It is a bitter experience for me to be informed now that centrally ordered Japanese war criminality of the most disgusting kind was kept secret from the court by the U.S. government."[108]

Critics argue that racism led to the double standard in the American postwar responses to the experiments conducted on different nationalities.[109] Whereas the perpetrators of Unit 731 were exempt from prosecution, the U.S. held a tribunal in Yokohama in 1948 that indicted nine Japanese physician professors and medical students for conducting vivisection upon captured American pilots; two professors were sentenced to death and others to 15–20 years' imprisonment.[109]

It just keeps going.

Separate Soviet trials

Although publicly silent on the issue at the Tokyo Trials, the Soviet Union pursued the case and prosecuted 12 top military leaders and scientists from Unit 731 and its affiliated biological-war prisons Unit 1644 in Nanjing and Unit 100 in Changchun in the Khabarovsk war crimes trials. Among those accused of war crimes, including germ warfare, was General Otozō Yamada, commander-in-chief of the million-man Kwantung Army occupying Manchuria.

Official silence during the American occupation of Japan

As above, during the United States occupation of Japan, the members of Unit 731 and the members of other experimental units were allowed to go free. On 6 May 1947, Douglas MacArthur, the Supreme Commander of the Allied Forces, wrote to Washington in order to inform it that "additional data, possibly some statements from Ishii, can probably be obtained by informing Japanese involved that information will be retained in intelligence channels and will not be employed as 'war crimes' evidence".[6]

According to an investigation by the The Guardian, after the end of the war, under the pretense of vaccine development, former members of Unit 731 conducted human experiments on Japanese prisoners, babies and mental patients, with secret funding from the American Government.[114] One graduate of Unit 1644, Masami Kitaoka, continued to perform experiments on unwilling Japanese subjects from 1947 to 1956. He performed his experiments while he was working for Japan's National Institute of Health Sciences. He infected prisoners with rickettsia and infected mentally-ill patients with typhus.[115] As the chief of the unit, Shiro Ishii was granted immunity from prosecution for war crimes by the American occupation authorities, because he had provided human experimentation research materials to them. From 1948 to 1958, less than five percent of the documents were transferred onto microfilm and stored in the US National Archives before they were shipped back to Japan.[116]

Ultimately, inadequate scientific and engineering foundations limited the effectiveness of the Japanese program.[122][123] Harris speculates that US scientists generally wanted to acquire it due to the concept of forbidden fruit, believing that lawful and ethical prohibitions could affect the outcomes of their research.[124]

So glad US nuked civilians so they could have sole occupation of Japan.