In Bury My Heart at Wounded Knee there are vigorous descriptions of U.S. soldiers mutilating Native Americans (private parts, children). To what extent is this true and why was no one ever prosecuted?
While awaiting an answer, check out this response from an earlier discussion, where /u/The_Alaskan makes the relevant point that, despite what occurred, it was 40 years before the media and public began discussing the event as a "massacre."