(CNN) -- The sheriff of Snohomish County, Washington, points at a pile of muddy rubble and describes what he sees.
"You can tell from the debris this appears to be part of somebody's kitchen, the inside of the house," said Ty Trenary.
Authorities in rural Washington worked feverishly Sunday to find survivors after a landslide tore through the area north of Seattle, killing four people and injuring seven. At least 18 people remained unaccounted for, a fire chief said.
"Total devastation. I mean, it's just unbelievable. It reminds me of what a tornado looks like when it's touched the ground," the sheriff said.
The Saturday landslide, which encompassed about one square mile, was caused by groundwater saturation tied to heavy rainfall in the area over the past month.
Trenary said rescue crews were working both sides of the slide. Authorities were also using helicopters to try to identify heat signatures, or people who may have been able to get free, he said.
But such work is complicated.
While there's a tremendous effort to rescue people who may be trapped, Snohomish County Fire District 21 Chief Travis Hots said Sunday that the rescue operation must be focused on keeping responders safe because the area is highly unstable.
The mud flow is like quicksand, he said. Responders suspect there are survivors, but it is "far too dangerous" to get rescuers to those people. The landslide is 15 feet deep in some places.
On Saturday rescuers dug through the rubble while survivors were crying for help underneath the debris, Hots told reporters. 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," he said, and rescuers had to back off.
"Mother Nature holds the cards here on the ability of ground personnel to enter the slide area. It is essentially a slurry," Washington Gov. Jay Inslee told reporters Sunday.
He called the rescue operation "aggressive."
"Every human endeavor ... is being explored here to rescue and find their loved ones," the governor said.
Inslee added that some rescuers had gotten "caught ... up to their armpits" in the slide and "had to be dragged out by ropes."
John Lovick, a county executive, addressed a reporter's question about whether voices were still being heard.
"We were told that there were noises in that area," Lovick said, stressing that fire chief Hots had decided that it was "too risky" to place rescuers in that area. "We are not hearing any reports of people hearing voices today or after last night."
Inslee said he plans to talk with federal officials about Federal Emergency Management Agency relief, adding that he'd never before seen firsthand the kind of devastation he witnessed by surveying the area by helicopter.
As of Sunday morning, Harborview Medical Center spokeswoman Susan Gregg told CNN the hospital was treating five patients: a 25-year-old woman in satisfactory condition; a 6-month-old boy in critical condition in the intensive care unit; a 37-year-old man in serious condition in the intensive care unit; a 58-year-old man in serious condition in the intensive care unit; and an 81-year-old man in critical condition in the intensive care unit.
At least six houses were destroyed in the landslide, and as many as 16 were damaged, the Snohomish County Sheriff's Office 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.
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 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."
Agencies including the state transportation and emergency management departments, the U.S. Navy and fire departments across Snohomish County responded.
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 anywhere near the Stillaguamish River downstream of the slide.
"Water could break through at any moment," the sheriff's office tweeted.