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 yesterday -- 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.
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.
In the disaster zone, helicopters circled above submerged houses in a search for survivors in the western US state, with hundreds still missing.
About 350 people were unaccounted for in Larimer County alone, from where about 475 people were evacuated, according to the sheriff's office.
In neighbouring Boulder County, 231 people were unaccounted for, according to CNN, though authorities cautioned that the numbers were fluctuating.
"It is no doubt an epic event," Weld County Commissioner Sean Conway told The Denver Post. "It is a once in 500 years or 1,000 years situation."
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