So far, officials have confirmed that eight people were killed and eight people were injured, but 18 people remain missing from the area.
King 5 News in Seattle is reporting the official death toll could rise when officials hold a news conference at 9 a.m. PT Monday.
The 2.6-square-kilometre slide destroyed about 30 homes when a wall of mud and debris swept through the community of Oso, blocking about 1.5 kilometres of Highway 530.
Search and rescue teams took to the air in helicopters and the ground on foot on Sunday looking for anyone who might still be alive, but hope of finding any more survivors from the massive mudslide waned as crews worked through the night into Monday.
"We didn't see or hear any signs of life out there today," Snohomish County Fire District 21 Chief Travis Hots said. "It's very disappointing to all emergency responders on scene."
Authorities believe the slide was caused by ground made unstable by recent heavy rainfall.
Washington Gov. Jay Inslee described the scene as "a square mile of total devastation" after flying over the disaster area midday Sunday. He assured families that everything was being done to find their missing loved ones.
The slide blocked the North Fork of the Stillaguamish River. With the water pooling behind the debris, authorities worried about downstream flooding and issued an evacuation notice Saturday. The water had begun to seep through the blockage Sunday afternoon, alleviating some concerns.
Snohomish County officials said Sunday that residents could return home during daylight hours. Even though the evacuation had been lifted, Inslee urged residents to remain alert.
The National Weather Service has issued a flash flood watch for Snohomish County through Monday afternoon.