Fleft hundreds unaccounted for.
Raging flood waters 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.
Larimer County Sheriff Department said on Twitter that the elderly woman was "missing presumed dead," after flood waters destroyed her house, and officials believe the toll is likely to climb.
"We do estimate that potentially there will be other loss of life," said Nick Christensen, executive officer of the sheriff's office, during a Saturday afternoon press conference.
Helicopters circled above submerged houses in a search of survivors in the western US state, with the whereabouts of hundreds still unknown.
Officials said part of the challenge of pinning down who was in need of help, is that communications systems in many areas were not working, after many cell phone towers were damaged in the flooding.
Meanwhile, new flash floods were expected to inundate the area, which thousands were forced to evacuate.
Officials said there were widespread power outages as streets became raging rivers after the state received months' worth of rain 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.
Search and rescue teams are being deployed to assess the situation and contact stranded residents, officials said.
The US National Guard provided seven helicopters on Saturday to help get people out of danger. About 1 200 residents were being pulled out of the Pinewood Springs area, the sheriff's office said.
But many others are still awaiting pick up, and authorities said that rescue for some could take days.
