On February 1, 1865, President Abraham Lincoln signed the 13th Amendment to the US Constitution. As the amendment made explicit, “neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.”
Read more about it: Tsesis, Alexander. The Promises of Liberty The History and Contemporary Relevance of the Thirteenth Amendment. New York: Columbia University Press, 2010. (ebook)