HILDALE, Utah — When the thunderstorms hit, it only took a few minutes for Short Creek to turn from a trickle into a lethal torrent, ripping through this community.  “A roaring river, just a wall of mud and debris, ripping up trees,” said Pamela Black, 63, whose house overlooks the creek. “We knew it would flood, because it always does when there’s a downpour, but never anything like this.”  At least 12 people died in a flash flood Monday, and one other was still missing late Tuesday. The victims, all women and children, were trying to drive over a gully when their cars were swept away.  “It is dry most any day of the year, but in this instance there was a small flood starting in that channel,” said Kevin Barlow, assistant fire chief of Hildale, a small city on the Arizona border. “They hesitated and watched the flood for a minute and were backing up when a huge 300- to 400-foot-wide flash flood came around and even behind them.”  The names of the dead and missing were not immediately released, but Hildale residents said that the victims, like many people in the area, were members of the Fundamentalist Church of Jesus Christ of Latter-day Saints, or F.L.D.S., a polygamous sect that split from the Mormon Church in the early 20th Century. The longtime sect leader, Warren S. Jeffs, was convicted in Texas in 2011 of child sexual assault, and is serving a life sentence in prison.  The victims, in two vehicles, belonged to two families that had gone to Maxwell Park but decided to seek shelter when the storm struck, Philip Barlow, the mayor of Hildale, said at a news conference. Seeming stunned, Mr. Barlow explained that the victims had been driving on Canyon Street in the northeast part of the city and were crossing a bridge when a fast-moving surge of water and debris rose from the creek below and carried them off.  The vehicles were found a quarter-mile away, and some of the bodies were not found until Tuesday because they were carried far downstream into Arizona (one was five miles away). Three women and six children — the youngest 4 years old — were confirmed dead, and one woman and three children were missing, officials said. Three children were rescued from the vehicles; one was hospitalized and released Tuesday afternoon, the mayor said.  Flash flood warnings from the National Weather Service had been broadcast, but many residents may not have seen them. Warnings were also sent by text message, but Mayor Barlow said that alert did not reach his phone until after the deluge.  A light but steady rain saturated the ground on Monday, and then two strong thunderstorms swept through in the late afternoon, dropping two and a half inches of rain in just two hours, said Lisa Verzella, a Weather Service meteorologist in Salt Lake City.  “There were trees coming down our road and huge rocks,” Irene Jessop, 22, said. “They’ve cleared all the major debris, but there’s about a foot of mud still on top of the road. The street we live on is very prone to flooding, and it happens every year, but never to this magnitude.”  Isaac Wyler, 49, returned on Monday night to his home in Colorado City, Ariz., adjacent to Hildale, to find his neighborhood upended.  “I saw, two streets down, a dump truck completely upside down,” he said. “I saw retaining-wall blocks, a couple of feet of solid concrete, about a half a mile from where it started.”  One video shot by residents shows a brick-red stream, swollen far past its banks, carrying off several cars, and another shows emergency workers wading through hip-deep water to pull women, in the distinctive long dresses of the F.L.D.S., from the windows of a stranded car.  Emergency crews and about 500 volunteers searched for survivors and cleared debris Tuesday, each team assigned to a segment of the creek. Recovery work was hampered by wreckage that made many streets and driveways impassable, and clogged the culverts that normally allow water to pass under roads. There was also the threat posed by another round of thunderstorms, and Mayor Barlow said the authorities had stationed people farther up the canyons to radio warnings of any new floods.  Workers from multiple agencies in four counties in Arizona and Utah — Washington, Mojave, Kane and Coconino — joined the search Tuesday, using swift-water safety gear, and search-and-rescue and cadaver-sniffing dogs. National Guard troops were also called out, and were expected to begin arriving Tuesday night.  Colorado City and Hildale were founded a century ago by a forerunner group to the F.L.D.S. and have long been known as polygamist havens where church leaders wielded enormous power over residents. Most of the land in both cities has historically been owned by the United Effort Plan Trust, created by F.L.D.S. leaders who effectively controlled municipal government, including the police. The cities were remote and small — their combined population is less than 8,000 — and for generations, both states largely left them alone.  In the past decade, though, the authorities have taken a more aggressive stance toward the church, including prosecutions of Mr. Jeffs and other leaders. The state of Utah took over the trust, and has been working to strip it of some of its power by establishing individual ownership of homes.  Ian Lovett reported from Hildale, Utah, and Richard Perez-Pena and Christine Hauser from New York.
At least 12 people were killed when flash floods roared through a Utah canyon, sweeping away the cars they were riding in, the authorities said.