Eight people were injured, including a six-month-old infant, when mud, water and rock smashed into the rural town of Oso, northeast of Seattle, on Saturday, police and firefighters said. Six homes and much of a highway were destroyed.
"There is a full-scale, 100-percent, aggressive rescue effort," Washington state Governor Jay Inslee told reporters, adding that helicopters, hovercrafts and rescue personnel had rushed to the scene.
"There's no missing piece in this rescue effort," he added, saying he had declared a state of emergency.
Snohomish County spokeswoman Shari Ireton told AFP: "We are able to confirm we know that 18 people who may have been in the area are unaccounted for. We do not have identification for those people that we are releasing at this time."
The search and salvage operation in the mud-covered terrain was extremely dangerous, the unstable, rain-saturated surface putting emergency personnel at risk of further mudslides.
"Some (rescue workers) went in and got caught literally up to their armpits" in mud and had to be pulled out themselves, Inslee said.
Some survivors had been hoisted to safety in the air, as others described the landslide as huge.
"All of a sudden there was a wall of mud" about 25 feet (almost eight meters) high, Robin Youngblood told The Seattle Times newspaper.
"Then it hit and we were rolling. The house was in sticks. We were buried under things, and we dug ourselves out."
The landslide destroyed her home, she said.
Three people were confirmed dead in the hours after the landslide hit, and on Sunday the Snohomish Sheriff said on Twitter that the death toll had risen to four.
One survivor was driving on the highway just as the mudslide hit. "I was coming down the hill and I just saw the darkness," Paulo Falco said on a local news report. "And everything was gone."
Rain has been especially heavy in the Cascade Mountains region in the past weeks, officials said. The forecast is for a break in the rain Sunday, but 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.
However, Inslee said geological surveys of the site suggested the water would be safely diverted over the coming days and evacuated residents were being allowed back home.
Senator Patty Murray 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.