Rescuers rushed by land and by air to evacuate Colorado residents stranded by epic mountain flooding as debris-filled rivers became muddy seas that extended into towns and farms kilometres from the Rockies.
Four people have been confirmed dead since the harrowing floods began on Friday. And hundreds of others have not been heard from in the flood zone, which now covers nearly 11,655sq km.
Some of those unaccounted for may be stranded or injured. Others might have got out but not yet contacted friends and relatives, officials said.
But police expected to find more bodies as the full scope of damage becomes clear.
A woman was missing and presumed dead after witnesses saw floodwaters from the Big Thompson River destroy her home in the Cedar Cove area, Larimer County sheriff's spokesman John Schulz said.
"I expect that we're going to continue to receive reports of confirmed missing and confirmed fatalities throughout the next several days," he said.
National Guard helicopters and truck convoys broke through to paralysed canyon communities where thousands of stranded residents were eager to escape the Rocky Mountain foothills. But not everybody was willing to go. Dozens of people in the isolated community of Jamestown wanted to stay to watch over their homes.
Authorities warned residents who chose not to leave that they might not get another chance for a while and should be prepared to endure weeks without electricity, running water and basic supplies.
"We're not trying to force anyone from their home. We're not trying to be forceful, but we're trying to be very factual and definitive about the consequences of their decision, and we hope that they will come down," Boulder County Sheriff Joe Pelle said.
Special education teacher Brian Shultz, 38, was torn about leaving his Jamestown home as he sat outside a makeshift shelter at a high school.
"I was thinking about staying. I could have lasted at least a year. I have a lot of training in wilderness survival," he said. About 10 of their neighbours declined to evacuate, his wife, Meagan Harrington, said. "They said they wouldn't force you, but it was strongly encouraged," she said.
Shultz' eyes welled with tears behind his sunglasses as he compared his situation to that of his neighbours.
"At least all of our stuff's there and will be there when we get back. The people right by the river, their houses were washed away. Other people thought their houses were going to be okay, and then they started to go. It's just really devastating."
Across the foothills, rescuers made progress against the floodwaters. But they were still unable to go up many narrow canyon roads that were either under water or washed out.
The military put more troops on the ground and helicopters in the air to aid in the search-and-rescue effort. More rain was forecast.
More than 1750 people and 300 pets had been evacuated over two days, National Guard Lieutenant Colonel Mitch Utterback said.
A helicopter taking Colorado Governor John Hickenlooper on a tour of the flooded areas stopped to pick up four stranded people and their two pets.
Above the plains of Larimer County, rescue crews planned to fly as many missions as possible while skies were clear.
Crews used inflatable boats to pick up families and pets from farmhouses.
For those awaiting an airlift, Guardsmen dropped food, water and other supplies into the winding, narrow canyons.