LOS ANGELES - Emergency workers in a massive effort to rescue stranded flood victims in Colorado, where more than 500 people are still unaccounted for, braced for a fresh pounding from storms Sunday.
Officials said efforts to locate those in need of help were hampered by flood damage to many cell phone towers.
New flash floods were expected to inundate the area, which thousands were forced to evacuate.
Raging floodwaters in the city of Boulder, already confirmed to have killed at least four people, apparently claimed the life of a fifth on Saturday - a 60-year-old woman swept away in the torrent.
The Larimer County Sheriff's Office said on Twitter that the woman was "missing presumed dead," after floodwaters destroyed her house, and officials warned the toll will likely climb further.
"There might be further loss of life," Boulder County Sheriff Joe Pelle told reporters. "It's certainly a high probability... With an army of folks and an air show, 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.
And 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.
In the disaster zone, helicopters circled above submerged houses in a search for survivors in the western US state, with hundreds still missing.