Flash floods have killed 12 people near Utah's border with Arizona when a "large wall of water and debris" triggered by heavy rain pounding nearby canyons swept them away in their cars, officials said on Tuesday.
In Zion National Park to the north, four people died and three were missing after going to explore canyons on Monday, officials said.
"Another tragedy for our state. Reeling right now," Utah Lieutenant Governor Spencer Cox said on Twitter.
In the small city of Hildale, Utah, hundreds of volunteers were helping search for one person still missing, Washington County officials said, after floodwaters swept through streets.
"It was an act from God," Hildale Mayor Phillip Barlow told reporters of Monday's tragedy, according to the Deseret News. "This is something we can't control ... It happened too fast."
Crews have been searching the banks of Short Creek amid sporadic showers, while contractors using heavy equipment have worked to clear thousands of tons of mud and debris. The National Guard has been called in to help with the cleanup.
"In the flash flooding two occupied vehicles were hit by a large wall of water and debris at the Canyon Street Maxwell Crossing and were carried into the flood," Washington County officials said in a statement.
An advisory for people to boil their water was issued late Tuesday due to damage to the water system.
Utah Governor Gary Herbert said he was "heartbroken" over the loss of life.
Hildale, home to fewer than 3,000 people, is twinned with Colorado City, across the border in Arizona.
Both cities are home to the polygamous Fundamentalist Church of Jesus Christ of Latter-day Saints. That sect is not affiliated with the Salt Lake City-based Church of Jesus Christ of Latter-day Saints, which renounced polygamy in 1890.
"It's the most terrible thing," said Ross Chatwin, a Hildale resident who is not affiliated with the sect, whose members tend to have little contact with outsiders.
"There has been no confrontation," Chatwin told the KSL-TV channel of efforts by non-members to help. "They are allowing everyone to come in freely."
Separately, four people died and three were missing after going to explore canyons at Zion National Park, less than 20 miles (32 km) north of Hildale, before Monday's heavy rains, the National Park Service said.
The search for the missing hikers has been hampered by concern about flooding and other hazards, it said.
(Reporting by Peg McEntee; Additional reporting by Victoria Cavaliere in Los Angeles and Daniel Wallis in Denver; Editing by Mohammad Zargham and Eric Beech)