By Ashley Fantz, Joe Sutton and Janet DiGiacomoAt least 18 people are unaccounted for following a landslide north of Seattle, the fire chief said at a press conference Sunday.
The landslide on Saturday affected an area of about a square mile and resulted in three deaths, Snohomish County Fire District 21 Chief Travis Hots said. Seven people have been taken to the hospital.
The landslide was caused by groundwater saturation tied to heavy rainfall in the area over the past month, authorities said.
While there's a tremendous effort to rescue people who may be trapped, Hots told reporters that the rescue operation must be focused Sunday on keeping responders safe because the area is highly unstable.
There is mud flow that is like quicksand, he said. Responders suspect there are survivors but it is "far too dangerous" to get rescuers to those people.
On Saturday rescuers dug through the rubble while survivors were crying for help underneath the debris, Hots confirmed. Hots said that rescuers heard voices around 11:30 p.m. and considered trying to reach the possible survivor or survivors, but "the mud was too thick and deep," Hots said, and rescuers had to back off.
For the time being, helicopters are surveying the area so that an assessment can be made as to when and how it will be safe for responders to attempt to help people.
At least six houses were destroyed in the landslide and as many as 16 were damaged, the Snohomish County Sheriff's Office reported earlier Sunday.
Seven adults and a 6-month-old boy were rescued and sent to local hospitals, deputies said.
The landslide affected the towns of Oso, a remote community of about 180, and Darrington, a town of about 1,350 people. The landslide cut off State Road 530 to Darrington. Part of the Stillaguamish River also was blocked, and residents were warned of possible flooding both upstream and downstream of the collapse.
The Washington State Patrol provided photos that showed floodwaters and sprawling debris covering a rural patch of the road, framed by woodlands and snow-capped mountains.
Earlier Sunday, Harborview Medical Center in Seattle reported that five patients had been airlifted and were in its care. Three of those -- including the baby, a 58-year-old man and an 81-year-old man -- were in critical condition Saturday night, according to spokeswoman Susan Gregg.
The first reports of the landslide came in around 10:45 a.m. Saturday (1:45 p.m. ET), the sheriff's office said. CNN first learned of the landslide via Twitter.
The county later said "we strongly recommend" that those living in the north fork of the Stillaguamish River flood plain, from Oso to Stanwood, "evacuate your home immediately."
"We are working on establishing shelters for those who have nowhere to go," county spokeswoman Rebecca Hover said in a statement. "Until then, people should get to higher ground as soon as possible."
"The situation along the Stillaguamish River is extremely dangerous," Washington Gov. Jay Inslee said in a statement Saturday. "Anyone along the flood plain of the Stillaguamish between Oso and Stanwood should leave the area before dark due to risk of catastrophic flooding."
A number of agencies have responded, including the state transportation and emergency management departments, the U.S. Navy and fire departments across Snohomish County.
Upon arriving at the scene, firefighters and state troopers heard calls for help, trooper Mark Francis said.
The Snohomish sheriff warned people to stay clear of trestles or bridges or anywhere near the Stillaguamish River downstream of the slide.
"Water could break through at any moment," the sheriff's office tweeted.
CNN's Michael Martinez and AnneClaire Stapleton contributed to this report.
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