The McKinney police said, in a statement, that they were called to respond to the Craig Ranch North Community Pool for a report of “a disturbance involving multiple juveniles at the location, who do not live in the area or have permission to be there, refusing to leave.” They added that additional calls reported fighting, and that when the crowd refused to comply with the first responding officers, nine additional units were deployed.

The mayor, Brian Loughmiller, described himself as “disturbed and concerned,” and the police chief vowed “a complete, and thorough, investigation.”

Like many flourishing American suburbs, McKinney has struggled with questions of equity and diversity. The city is among the fastest-growing in America, and its residents hail from a wide range of backgrounds. Formal, legal segregation is a thing of the past. Yet stark divides persist.

In 2009, McKinney was forced to settle a lawsuit alleging that it was blocking the development of affordable housing suitable for tenants with Section 8 vouchers in the more affluent western portion of the city. East of Highway 75, according to the lawsuit, McKinney is 49 percent white; to its west, McKinney is 86 percent white. The plaintiffs alleged that the city and its housing authority were “willing to negotiate for and provide low-income housing units in east McKinney, but not west McKinney, which amounts to illegal racial steering.”

All three of the city’s public pools lie to the east of Highway 75. Craig Ranch, where the pool party took place, lies well to its west. BuzzFeed reports that the fight broke out when an adult woman told the teens to go back to “Section 8 housing.”

Craig Ranch North is the oldest residential portion of a 2,200 acre master-planned community. “The neighborhood is made up of single-family homes,” says the developer’s website, “and includes a community center with two pools, a park and a playground.” Private developments like Craig Ranch now routinely include pools, often paid for by dues to homeowners’ associations, and governed by their rules. But that, in itself, represents a remarkable shift.

At their inception, communal swimming pools were public, egalitarian spaces. Most early public pools in America aimed more for hygiene than relaxation, open on alternate days to men and women. In the North, at least, they served bathers without regard for race. But in the 1920s, as public swimming pools proliferated, they became sites of leisure and recreation. Alarmed at the sight of women and men of different races swimming together, public officials moved to impose rigid segregation.

As African Americans fought for desegregation in the 1950s, public pools became frequent battlefields. In Marshall, Texas, for example, in 1957, a young man backed by the NAACP sued to force the integration of a brand-new swimming pool. When the judge made it clear the city would lose, citizens voted 1,758-89 to have the city sell all of its recreational facilities rather than integrate them. The pool was sold to a local Lions’ Club, which was able to operate it as a whites-only private facility.