The massacre was ignited by the arrest of a black shoeshiner, accused of assaulting a 17-year-old white elevator operator. After he was taken into custody, angry local whites gathered outside the courthouse where he was being held. Rumors that he had been lynched alarmed the local black population, some of whom showed up at the courthouse with weapons. Shots were fired and twelve people were killed: ten white and two black.
After news of these deaths spread through the city, white mobs rampaged through the black neighborhood that night and morning, killing men and burning and looting stores and homes. Martial law was declared June 1 and Oklahoma National Guard troops took control of the streets.
The riots left about 10,000 black people homeless.
Sources:
The Destruction of Black Wall Street by Devin M.L. Andrews.