KKK

The Ku Klux Klan (KKK), founded in 1865, extended into almost every southern state by 1870. A group including many former Confederate veterans founded the first branch of the Ku Klux Klan as a social club in Pulaski, Tennessee, in 1865. In the 1920's, the Klan during its revival exceeded 4 million people nationwide.

They dedicated themselves to an underground campaign of violence against Republican leaders and voters in an effort to restore white supremacy in the South. They were joined by similar organizations such as the White Brotherhood and the Knights of the White Camelia. These Klans all had one goal, they wanted to send a message to the blacks. About 10 percent of the black legislators elected during the constitutional conventions became victims of violence during Reconstruction, and seven who were killed. White Republicans and black schools and churches were big targets for the Klans.

The Klan members would wear masks and dress up in long white robes usually with their hood on. Klan activity usually happened in the regions of the South where blacks were a minority or a small majority of the population and would always go out at night so their identity couldn't be easily defined. In January 1871, South Carolina had 500 masked men attacking the Union county jail which led to the lynching eight black prisoners.

https://www.history.com/topics/reconstruction/ku-klux-klan