True, they gave a number of general apologies, but never on this particular matter. Unit 731 was completely pushed under the carpet, the people involved were given immunity by US in exchange for their knowledge on making bio-weapons. During the US occupation and later some ex-members of Unit 731 even continued to conduct medical experiments on unwilling human subjects, this time Japanese, which was really what triggered journalists to start digging into the story and that's how public learned about it. However, until 2002. Japan officially never even admitted conducting biological warfare, and no data on Unit 731 was ever released (officially Japan has no documentation about it at all). And Japan is not the only to blame here, US played the major role in covering up this story, just like in case of Klaus Barbie and many other nazi killers who they protected in exchange for information or specific skills.