This question has been inspired by the recent court case involving Guatemalan victims of medical testing involving the injection of STIs in people's spines, eyeballs, etc, and the observation of the infection of the victims families for epidemiological purposes. Those studies were in the 40's and 50's. Were medical experiments done on slaves and indigenous people? Did America have a residential schooling system which involved hundreds of mysterious deaths, like our neighbors to the north? Did the Civil War involve any massacres?

Other than genocide and slavery, What are the most profound and twisted atrocities of pre WWII American History?

Edit: in particular I am interested in the history of atrocities involving human experimentation. less specifically, all actions which would be described as war crimes in modern times if they were done by unfriendly forces. (It is most important to me now to develop an understanding of the frequency and breadth of this type of behavior more than the depths of it's depravity, So simple lists are welcome)