At least 16 people were killed by flash flooding in Utah, including 12 women and children who died after a torrent of water Monday evening swept through a tiny community on the Utah-Arizona border that is home to members of a polygamous sect.

The other four found dead were hikers in nearby Zion National Park.

In the town of Hildale, Utah, which had been drenched with heavy rains, a surge of water came roaring out of a canyon, washing away a van and sport-utility vehicle that were filled with women and children who were returning from a city park, said Kevin Barlow, the town’s assistant fire chief.

“I’ve never seen the likes of that,” Mr. Barlow said. “There are flash flood warnings almost every day during monsoon season, but this particular storm, it just came down fast and furious.”

Hildale and the neighboring community of Colorado City, Ariz., are best known for being home to members of the Fundamentalist Church of Jesus Christ of Latter-Day Saints. The towns are just over 300 miles from Salt Lake City.