The riot began over a Memorial Day, weekend, 1921; after 19-year-old Dick Rowland, a black skinned shoeshiner, was accused of assaulting Sarah Page, the 17-year-old white skinned elevator operator of the nearby Drexel Building. Rowland says he tripped on the elevator threshold and reflexively grabbed her arm to steady himself. Initially he was arrested for assault. After all the rioting died down, young Sarah Page later decided not to press charges against Rowland.
Here's how Wikipedia details the next events....
After he was taken into custody, rumors raced through the black community that he was at risk of being lynched. A group of armed African-American men rushed to the police station where the young suspect was held, to prevent a lynching, as a white crowd had gathered. A confrontation developed between black and white people; shots were fired, and twelve people were killed, ten white and two black.
As news of these deaths spread throughout the city, mob violence exploded. Thousands of white people rampaged through the black community that night and the next day, killing men and women, burning and looting stores and homes. About 10,000 black people were left homeless, and property damage amounted to more than $1.5 million in real estate and $750,000 in personal property ($31 million in 2018).
Tulsa race riot of 1921, took place on May 31 and June 1, 1921, when a mob of white citizens attacked residents and businesses of the African-American community of Greenwood in Tulsa, Oklahoma. This is considered one of the worst incidents of racial violence in the history of the United States. The attack, carried out on the ground and by air, destroyed more than 35 blocks of the district, at the time the wealthiest black community in the U.S.