The confirmed death toll remains at eight after the massive landslide slammed "like a freight train" into a mountainside community, said Snohomish County emergency management chief John Pennington.
The wall of mud, rocks and trees smashed into the rural town of Oso, northeast of Seattle, on Saturday. Three people were reported dead shortly after and another body was pulled from the rubble earlier on Sunday.
Travis Hots, chief of the regional Snohomish County Fire districts, announced the higher toll at an area community meeting.
The field of rubble is about 2.4 kilometers (1.5 miles) across and some four to six (15-20 feet) deep in areas, The Seattle Times reported.
No signs of life
Rescuers reported hearing voices calling for help on Saturday, but Hots told reporters that they "didn't see or hear any signs of life" on Sunday.
John Pennington, head of the county Emergency Management department, said that rescuers will continue searching for survivors overnight Sunday to Monday.
"Resources are coming in that allow us to conduct night operations," Pennington said at the briefing. "There are boots that are on the ground that are really working to continue operations 24/7."
Six homes and much of a two-lane highway reaching the area were destroyed, while as many as 16 other homes were damaged, the Snohomish County Sheriff's Office said.
Washington state Governor Jay Inslee, who declared a state of emergency for the area, told reporters there is "a full-scale, 100-percent, aggressive rescue effort" going on, adding that helicopters, hovercrafts and rescue personnel had rushed to the scene.
"There's no missing piece in this rescue effort," he said.
The muddy area was so unstable that some rescue workers "went in and got caught literally up to their armpits" and had to be pulled out themselves, Inslee said.
'Like a freight train'
People injured in the landslide include a six-month old infant and an 81 year-old man.
"It sounded like a freight train," landslide witness Dan Young told Komo4News. "In just 35 to 45 seconds it was over." Young's home survived but is flooded.
Rain has been especially heavy in the Cascade Mountains region in the past weeks. While there was a break on Sunday, the forecast is for more heavy downpours throughout the week.
Authorities were keeping careful watch on a nearby dam, over fears pressure from the flooded river behind it could wash it away, inundating downriver communities.
Patty Murray, who represents Washington in the US Senate, gave assurances that federal resources would be made available, as she offered thanks to rescue workers and her prayers to the families of the ravaged community.