Rescuers in the US state of Arkansas are searching for more than 20 people missing after floods swept through campsites in a national park.
At least 18 people were killed in the floods on Friday and more than 20 others taken to hospital.
Rescuers say bodies may have been washed away and the search for the missing could take days if not weeks.
Teams are using helicopters, horses and canoes to scour the mountainous area in the state's south-west.
River levels rose as fast as 8ft (2.4m) an hour after heavy rains on Friday morning, triggering a wall of water which tore through the campsites as many people were asleep.
Some campers described how they had to cling to trees and vehicles for hours to survive, as floods uprooted trees and tore asphalt from roads.
Six children were among the 18 confirmed fatalities.
The remains of destroyed tents and damaged log cabins were later seen lining the banks of the swollen rivers.
Dozens of people have so far been rescued.
The search operation is focused on campsites along the Little Missouri and Caddo rivers in the Ouchita Mountains.
As many as 300 people were believed to be in the area at the time but it was unclear how many were campers and how many local residents, officials say.
Rescuers admit they do not know how exactly many people they are looking for.
A log of campers in the Albert Pike Recreation Area, which has more than 50 pitches, was washed away with everything else when the floodwaters hit.
The campsite was packed with families on holiday and inquiries about 73 people who may be missing there were received on Friday.
"We haven't confirmed if they were at the campsite, but people have called because they believe a loved one may have been there and they can't locate them," Arkansas Department of Emergency Management spokesman Chad Stover told the Associated Press late on Friday.
"We still consider it a search-and-rescue operation for a little while longer."
Officials said that the rugged, heavy-forested terrain was hampering the operation.
They say that some of the victims could be trapped under fallen trees and rocks, and that the river water would not be clear enough to see through for several days.
"It's just a tangled mess," Tom Collins, a volunteer fire-fighter, was quoted as saying by the Associated Press.
The region includes a mix of campsites, hunting grounds and private homes.
Marc and Stacy McNeil of Marshall, Texas, survived by hauling their pick-up truck between two trees and standing in waist-deep water.
"It was just like a boat tied to a tree," Mr McNeil said, describing how the truck bobbed up and down.
They said the water kept rising throughout the night. By dawn the rain stopped, the water receded and they were able to walk to safety.