The US government spent $30 million over 7 years under Bush/Clinton Admins, aimed to find the War Crimes of NAZI and the Imperial Japanese government.
The final conclusion was made in the IWG report in 2007. They have thoroughly checked the entire declassified documents from the Central Intelligence Agency (CIA), the Office of Strategic Intelligence (OSI=the predecessor of CIA), and the General Headquarters of Allied Powers (GHQ).
Consequently, no evidence of human experimentation, forced prostitution (Comfort Women), the Nanking (Nanjing) massacre of civilians, or Unit 731 by the Japanese military was found.
The U.S. government at that time was demanded by a Chinese organization to find anything against Japan, and the U.S. was at odds with Japan at that time, there was no reason why IWG could not find them, except that they didn't exist in the first place.
https://www.archives.gov/files/iwg/reports/final-report-2007.pdf