The influx of black people into urban centers in the North and Midwest increased racial tensions in those cities. Between 1917 and 1923, racial disturbances erupted in numerous cities throughout the U.S., motivated by economic competition between different racial groups for industrial jobs. One of the first and most violent instances was a riot in East St. Louis, sparked in 1917. In the Red Summer of 1919, racially motivated mob violence erupted in 23 cities—including Chicago, Omaha, and Washington, D.C.—caused by competition for jobs and housing by returning World War I veterans of both races, and the arrival of waves of new European immigrants. Further unrest occurred in Tulsa in 1921, when whites attacked the black Greenwood community. David Colburn distinguishes two types of violence against black people up to 1923: Northern violence was generally spontaneous mob action against entire communities. Southern violence, on the other hand, took the form of individual incidents of lynchings and other extrajudicial actions. The Rosewood massacre, according to Colburn, resembled violence more commonly perpetrated in the North in those years.