With rain still falling and the flood threat still real, authorities called on thousands more people in the inundated city of Boulder and nearby towns to evacuate as rivers and creeks rose to dangerous levels.
The late-night reports from Boulder and the village of Eldorado Springs came as rescuers struggled to reach dozens of people cut off by flooding in Colorado mountain communities. Residents in the Denver area and other downstream communities were warned to stay off flooded streets.
The towns of Lyons, Jamestown and others in the Rocky Mountain foothills have been isolated by flooding and without power or telephone since rain hanging over the region all week intensified late Wednesday and early Thursday.
At least three people were killed and another was missing, and numerous people were forced to seek shelter up and downColorado's populated Front Range.
Boulder County spokesman James Burrus said 17 people were unaccounted for Friday, and officials planned to publicly release the names later.
"Unaccounted for doesn't mean missing. It means we haven't heard back from them," he said.
Two backpackers who were hiking the 14,259-foot Longs Peak became stranded when the weather turned, the mother of one of the backpackers said Friday.
Suzanne Turell and Connie Yang of York, Maine, last sent a text message at 9 a.m. Thursday with their GPS coordinates, but their cellphones have since gone dead, said Barbara Turell of Federick, Md.
They have a tent but do not have cold-weather gear, she said
"Those kids have been up there for two days now. They're not prepared for the cold weather," she said. "We're very concerned we may be in a long line of people needing assistance."
Late Thursday, warning sirens blared in Boulder and city officials sent notice to about 4,000 people living along Boulder Creek around the mouth of Boulder Canyon to head for higher ground, according to Boulder's Daily Camera newspaper.
The alert was prompted by rapidly rising creek levels caused by water backing up at the mouth of the canyon because of debris and mud coming off the mountainsides, the city Office of Emergency Management said.
The creek began to recede after midnight, but the conditions remained dangerous and a surprising amount of water was still flowing into the city's streets, Police Chief Mark Beckner told the Daily Camera after touring the damage.
The entire hamlet of Eldorado Springs, about 500 people, was urged to evacuate because of a flash flood and mudslide threat along South Boulder Creek, Burrus said.
Northwest of Boulder, the overflowing Vrain Creek cut the town of Longmont in half. Evacuation requests were issued for some neighborhoods, all major roads were closed, and several thousand homes and businesses were without power, he said.
Interstate 25 east of Loveland was closed in both directions Friday, state transportation officials said.
In Fort Collins, neighborhoods along the Cache La Poudre River were evacuated overnight, with the river expected to rise to nearly 2 feet above flood stage Friday morning, according to the National Weather Service.
City officials in Fort Collins closed bridges after water began topping Seaman Reservoir in the Poudre Canyon, The Denver Post reported. The city warned residents to stay clear of the river.
South of the historic Red Rocks Amphitheater, Jefferson County deputy sheriffs went door-to-door in Morrison and Kittridge, asking hundreds of residents to leave their homes as Bear Creek neared flood stage. The amphitheater was in no immediate danger.
In Lyons, residents took shelter on higher ground, including some at an elementary school. Although everyone was believed to be safe, the deluge was expected to continue into Friday.
"There's no way out of town. There's no way into town. So, basically, now we're just on an island," said Jason Stillman, 37, who was forced with his fiancee to evacuate their home in Lyons after a nearby river began to overflow into the street.
The Colorado National Guard began trucking people out of Lyons on Thursday evening.
To the north, residents along the Big Thompson Canyon in Larimer County, scene of the deadliest flash flood in state history, were also evacuated. The Big Thompson River flooded in 1976 after about a foot of rain fell in just four hours, killing 144 people.
Early Friday, the National Weather Service warned of more flash flooding in Loveland, according to the Post. NOAA reported that the Big Thompson River at Drake was more than 4 feet above its flood stage of 6 feet.
President Barack Obama signed an emergency declaration Thursday night, freeing federal aid and allowing the Federal Emergency Management Agency to coordinate disaster relief efforts.
Some of the flooding was exacerbated by wildfire "burn scars" that have spawned flash floods all summer in the mountains. That was particularly true in an area scarred by fire in 2010 near the tiny community of Jamestown and another nearColorado Springs' Waldo Canyon that was hit in 2012.
The University of Colorado canceled classes at least through Friday after a quarter of its buildings were flooded. Students in family housing near Boulder Creek were also forced to leave.
One person was killed when a structure collapsed in the tiny town of Jamestown northwest of Boulder. Another man drowned in floodwaters north of Boulder early Thursday, and a woman who was with him was missing.
The woman was swept away after the vehicle she was riding in got stuck in water. The man died after getting out of the vehicle to help her, Commander Heidi Prentup of the Boulder Sheriff's Office said.
To the south, Colorado Springs police conducting flood patrols found the body of 54-year-old Danny Davis in Fountain Creek on the west side of the city.
At least one earthen dam gave way southeast of Estes Park, the gateway to Rocky Mountain National Park. Water levels could rise downstream as authorities release more water to ease pressure on dams. With debris piling up near bridges, downstream farming areas including Fort Lupton, Dacono and Plateville were also at risk.
Associated Press writers Colleen Slevin, Steven K. Paulson and Thomas Peipert in Denver and Mead Gruver in Longmont contributed to this report.