Thirteen people have died after heavy rain pounded Kerr County, Texas, early Friday, leading to “catastrophic” flooding, the sheriff said.

Children are among the dead, said Lt. Gov. Dan Patrick, who has taken on the duties of acting governor.

There are several major camps in the area, and at one of them, Camp Mystic, about 23 of its 750 campers are unaccounted for, according to Patrick.

“That does not mean they’ve been lost — they could be in a tree, they could be out of communication. We are praying for all of those missing to be found alive,” Patrick said at a news conference.

Camp Mystic said in a letter to parents, “If your daughter is not accounted for, you have been notified. If you have not been personally contacted, then your daughter is accounted for.”

Up to 12 inches of rain has already fallen in Kerr County, about 60 miles north of San Antonio, with the rain ongoing through the evening.

Dozens of people were rescued Friday morning and search efforts are ongoing, Kerr County Judge Rob Kelly said.

“This came at night, when people were asleep in bed,” Kerrville Mayor Joe Herring said at an earlier news conference, overcome with emotion. “Please pray for our community.”

The Guadalupe River in Kerr County rose a whopping 26 feet in 45 minutes, Patrick said. The river reached its second-highest height on record, surpassing a 1987 flood level, the National Weather Service said.

While most residents should avoid traveling, “those near creeks, streams, and the Guadalupe River should immediately move to higher ground,” the sheriff’s office warned.

At one campground along the Guadalupe River, people filmed videos of themselves evacuating when they noticed flood water encroaching under their vehicle. Other videos showed cars and cabins submerged.

“I was up all night keeping an eye on weather alerts. 2am there was a river flood warning. The river was a good distance from our site, so we thought we would be fine,” one camper, Katie, told ABC News via Instagram. “At about 5:30am, we heard banging on our RV door, yelling for us to get out now. The Guadalupe River had risen rapidly. We got the kids in the truck and were prepared to leave our camper behind if need be. We were able to hook up our camper and get it out too.”

“The river was up to the back of our campsite when we left. The river was raging and you could hear trees and objects breaking,” she said. “It was a very scary situation, but we are very blessed by the workers at the park for waking people up and getting us out of there. Praying for everyone affected by the flooding.”

ABC News’ Mireya Villarreal, Kenton Gewecke and Helena Skinner contributed to this report.