Labor and immigrant conflict was a source of tensions that catalyzed as the East St. Louis riot of 1917. White rioters, many of them ethnic immigrants, killed an estimated 100 black residents of East St. Louis, after black residents had killed two white policemen, mistaking the car they were riding in for a previous car of white occupants who drove through a black neighborhood and fired randomly into a crowd of black people. White-on-Black race riots include the Atlanta riots (1906), the Omaha and Chicago riots (1919), part of a series of riots in the volatile post-World War I environment, and the Tulsa massacre (1921).