Earlier reports indicated that nine had been killed during the flooding.
According to Sgt. Brock Bentley of the Washington County Sheriff's Office, three additional bodies have been found, bringing the total to 12. Authorities say some victims could be as young as 4.
The flooding began Monday afternoon after heavy rains just north of Hildale, Utah and Colorado City, Arizona, sending a "wall of water" through the towns' streets.
"Unbeknownst to them, a flash flood had developed up in the canyon and it came rushing down, and it actually came around behind the vehicles and engulfed the vehicles," Hildale Mayor Philip Barlow said Tuesday, according to NBC News.
"This hit with a vengeance we haven't seen for some time," said Hildale's assistant fire chief, Kevin Barlow, according to NBC.
Hildale is home to a controversial, polygamous religious sect. The group's leader, Warren Jeffs, is currently serving a life sentence for sexually assaulting underage girls. Many in the community saw his imprisonment as a sign of religious persecution.
Roughly 150 emergency workers from county, state, and federal agencies have been involved in the rescue effort, as well as nearly 500 community volunteers.
Officials at Zion National Park, 20 miles north of Hilldale in Utah, confirmed that four were killed and three were missing after flash flooding in that location.
