In Hunt, Texas, where the Guadalupe River forks, more than seven inches of rain has fallen since Thursday afternoon—the highest total in that area since the early 1990s.

This led to a rapid rise of the Guadalupe River, which accelerated to over 29 feet before sunrise, the second-highest crest ever. The intensity of the rain took officials and residents by surprise.

The National Weather Service had issued a flood watch advisory for a broad swath of central Texas early Thursday afternoon, urging residents of several counties to be on alert for flood warnings.

It was followed by several urgent alerts the federal agency issued. Hundreds of emergency personnel were searching for stranded people, using 14 helicopters and ground crews who were struggling to navigate flooded roads, officials said. They warned that the death toll was likely to rise. “This is a tragic event.

It’s going to be a mass casualty event,” Freeman F. Martin, the director of the Texas Department of Public Safety, told reporters Friday afternoon. Mr. Patrick said Camp Mystic was contacting the parents of campers who remain unaccounted for. Camp Mystic, the Christian summer camp for girls on the Guadalupe River, is nearly a century old. Its facilities include a recreation hall that was constructed in the 1920s from local cypress trees. For those old enough to have lived through it, the flooding on Friday surfaced memories of a deadly swelling of the waters along the Guadalupe River on July 17, 1987. Our hearts and thoughts are with the survivors—Texas stands with you.