The water-soaked US state of Colorado saw more rainfall Sunday that threatened to slow the search for the 500 people unaccounted for after several days of massive flooding.
Among the hundreds missing, officials suggested many may simply not be able to call loved ones because of damage to cell phone towers or power outages.
"But we're still bracing," Colorado Governor John Hickenlooper said.
"There are many, many homes that have been destroyed. A number have been collapsed and we haven't been in them yet," he told CNN.
Some 500 people are unaccounted for, Hickenlooper added.
In addition, thousands have been evacuated after torrential downpours washed away roads and inundated communities, claiming at least six lives.
Rain began pelting the western state earlier this week, with Boulder especially hard hit, seeing 7.2 inches (18.3 centimeters) of precipitation in about 15 hours starting Wednesday night.
And with more downpours affecting already flood-ravaged areas, the situation could get even worse, Hickenlooper warned.
It was raining in Boulder, a university town, by early afternoon and a flash flood warning was in effect, The Weather Channel predicted. The same was true for Denver, it said.
With the ground already saturated, "that's going to just really magnify the problems we've had so far," Hickenlooper said in reference to Boulder County.
"We're still trying to evacuate people," he added, noting that almost 2,000 residents had been moved out of Boulder alone.
Liz Donaghey, a spokeswoman for the Boulder Office of Emergency Management, said poor visibility had grounded several US National Guard helicopters deployed to the area to help people get out of danger.
"At this point it is a big concern with the weather," she told CNN in a separate interview.
On Saturday, hail the size of peas or even marbles pummeled parts of the city of Aurora, according to local weather reports. A series of thunderstorms also struck the area.
Raging floodwaters, already presumed to have killed at least five people, apparently claimed the life of a sixth.
The latest apparent victim is an 80-year-old woman who was injured and couldn't get out of her home, the Denver Post quoted John Schulz, spokesman for the Larimer County sheriff's department, as saying.
"There might be further loss of life," Boulder County Sheriff Joe Pelle told reporters earlier. "It's certainly a high probability ... We're hoping to reach everyone as soon as possible."
But some additional help was on the way, with President Barack Obama declaring a major disaster in Colorado and ordering federal aid to support state and local efforts.
"Assistance can include grants for temporary housing and home repairs, low-cost loans to cover uninsured property losses, and other programs to help individuals and business owners recover from the effects of the disaster," the White House said.
Hickenlooper welcomed the assistance, saying the Federal Emergency Management Agency "has been terrific."
The Wyoming National Guard was helping the evacuation effort after Governor Matt Mead activated five UH-60 Black Hawk helicopters and 20 crew members, the state's military department said.
Some 1,200 residents were pulled out of the Pinewood Springs area by the National Guard and Fort Carson personnel, state authorities said on Twitter.
But many others were still awaiting rescue, which authorities said could take days for some.
Officials said there were widespread power outages as streets became raging rivers after the state received months' worth of rain in just a few days.
Pictures from helicopter cameras showed heavy rain had reduced the towns of Jamestown, Lyons and Longmont to little more than islands, with ready-to-eat meals being dropped to stranded, anxious residents below.
Residents' furry friends were also stranded by the torrential rains.
"Our victims' advocates told me tonight there were almost as many pets as people getting off the evacuation helicopters today," the Larimer County Sheriff's Office tweeted.
Weld County Commissioner Sean Conway called the widespread flooding an "epic event."
"It is a once in 500 years or 1,000 years situation," he told the Denver Post.
Hickenlooper concurred. "This is a heck of a storm," he said.