It was a stifling summer day in Chicago on July 27, 1919, with temperatures rising to 96°F — and tensions rising across the nation.

In the summer following the end of World War I, the American population was rapidly shifting. White immigrants from Europe were entering the country, leaving their impoverished homelands. Meanwhile, African Americans were fleeing the racism and poverty of the South for new opportunities in northern cities. For many, that meant Chicago. According to the Chicago Tribune, the city’s black population swelled from 44,000 in 1910 to 110,000 in 1920.

Those changes were the backdrop for “the largest outbreak of racial violence and domestic unrest” in the United States between the end of Reconstructionand the Civil Rights Era, says Peter Cole, a historian at Western Illinois University who is closely involved with new efforts to memorialize that period. Between April and November of 1919, white people in cities and towns across the country instigated race riots, attacking and often killing their black neighbors. The period would come to be known as Red Summer.

By the time the hottest weekend of the season came around, Chicago had already seen two dozen bombings of black residents, none of which was solved by the cops. So the city was already a powder keg of racial resentment when group of teenage boys decided that they wanted to go for a swim.

Eugene Williams, a black 17-year-old originally from Georgia, was among those boys. He and his friends had built a raft to play on in Lake Michigan. While most public accommodations in Chicago were not legally segregated, the beaches around Lake Michigan were the site of strong informal segregation. And as Williams and his friends swam and dove off their raft, they unknowingly drifted into the white section of the water.

Contemporary reports from the Chicago Evening Post state that white people in the water and on the beach grew agitated when they saw black teenagers in “their” section of the lake. A 24-year-old white man named George Stauber began throwing rocks at the boys. It is unclear exactly what happened next — some witnesses saw Williams get hit by the rocks and fall into the water; others say he ducked under to avoid getting hit. But accounts agree that Williams drowned in Lake Michigan as a result of Stauber’s racist harassment.

According to John Turner Harris, a friend of Williams who was interviewed decades later by historian William M. Tuttle, when it became clear that Williams was dead, a black police officer attempted to arrest Stauber. A white cop intervened and did not allow the arrest. An argument broke out between the two officers and Williams’ friends ran to notify other African Americans. According to contemporary reports by the Chicago Commission on Race Relations, a thousand black Chicagoans gathered following the news of the drowning. An altercation between that group and the police resulted in the death of James Crawford, a black man.

Emotions remained elevated for the rest of the day, eventually escalating into violence.