That May, a local aluminum plant brought in black workers to replace striking white ones. Soon, crowds of whites gathered downtown, at first protesting the migration, then beating blacks and destroying property. On July 1, a group of white men drove through a black neighborhood, firing a gun out their car window. (The perpetrators were never caught.) A few hours later, another car drove through the neighborhood. Black residents fired at it, killing two police officers.
On July 2, as news of the killings got out, white residents went tearing through black neighborhoods, beating and killing blacks and burning some 300 houses as National Guard troops either failed to respond or fled the scene. The official toll counted 39 black and eight white people dead, but others speculated that more than a hundred people died in what is still considered one of the worst incidents of racial violence in twentieth-century America. Afraid for their lives, more than six thousand blacks left the city after the riot.
via The St. Louis Area Has a Long History of Shameful Racial Violence | Mother Jones.