On May 31, 1921, a race riot broke out in Tulsa, Oklahoma. It was described as the worst incident of racial violence in American history with around 150-300 African Americans killed. From the article:
"During the Tulsa Race Massacre, which occurred over 18 hours from May 31 to June 1, 1921, a white mob attacked residents, homes and businesses in the predominantly Black Greenwood neighborhood of Tulsa, Oklahoma. The event remains one of the worst incidents of racial violence in U.S. history, and, for a period, remained one of the least-known: News reports were largely squelched, despite the fact that hundreds of people were killed and thousands left homeless...
...News Blackout
For decades, there were no public ceremonies, memorials for the dead or any efforts to commemorate the events of May 31-June 1, 1921. Instead, there was a deliberate effort to cover them up.
The Tulsa Tribune removed the front-page story of May 31 that sparked the chaos from its bound volumes, and scholars later discovered that police and state militia archives about the riot were missing as well. As a result, until recently the Tulsa Race Massacre was rarely mentioned in history books, taught in schools or even talked about."