They looked for him for I believe 7 years, but he was able to avoid capture. If he would have been captured in that time, he definitely would have been prosecuted. However, the political realities of the Cold War changed the situation. With China becoming communist, America really wanted Japan as an ally, but the war crime prosecutions were very unpopular in Japan. Therefore, the US pardoned anyone who hadn't been captured, and they drastically reduced the punishments of almost everyone else. Even some of the worst criminals ended up serving only 15 years, and many were let out even earlier. So basically, the US let these people off the hook to placate the Japanese population and win them over as an ally in the Cold War. I'll try to grab the book later today and flesh out this response. If you want to look at it yourself, it's at the very end of the book. It might even be in the Epilogue.

Source: Unbroken by Laura Hillenbrand