Denver:
More torrential rain grounded helicopters in the US state of Colorado on Sunday, slowing the search for up to 500 people unaccounted for after several days of massive flooding.
Among the 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.
Hickenlooper put the number unaccounted for state-wide at 500, but Boulder Sheriff Joe Pelle, in the hardest-hit region, said the numbers were changing rapidly: by mid-afternoon it had been cut to 326, from 431 earlier.
It was unclear if those figures included people still missing in other parts of the state.
"Finding the people who are unaccounted for is one of the highest priorities, and five teams of Boulder County Sheriff's Office detectives are dedicated full-time to this task," said Pelle, in an afternoon update posted by Boulder Office of Emergency Management.
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 said.
With the ground saturated, "that's going to just really magnify the problems we've had so far," he said in reference to Boulder County.
"We're still trying to evacuate people," Hickenlooper 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 casualty 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," Sheriff Pelle told reporters. "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.
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 agreed. "This is a heck of a storm," he said, but adding later on Twitter: "We've got a lot of broken roads & bridges, but we don't have any broken spirits."