the US struck a secret agreement with Ishii. In a memo to General Douglas MacArthur (1880 – 1964), commander of Allied forces in Japan, Washington recognized that although war crimes had been committed, the experiments led by Ishii and his colleagues were “almost incalculable and incredibly valuable to the United States.”
In exchange for the records of Unit 731’s experiments, the US granted Ishii and his assistants immunity. Ishii died, and his collaborators went on to have careers in prestigious universities and private laboratories.
The US was in possession of the data but likely wanted the scientists to provide context in the same way they wanted the experience of the nazi rocket scientists. But lifetime immunity and a cover up is a horrible way to deal with the problem.