The slide of mud, trees and rocks happened about 11 Saturday morning. Several people - including an infant - were critically injured and at least six houses were destroyed.
Snohomish County Fire District 21 Chief Travis Hots said at a news briefing late Saturday that searchers weren't giving up on finding more people alive.
"We have people who are yelling for our help, and we are going to take extreme risks," Hots said.
It wasn't clear how many people might still be trapped - or if more bodies might be discovered.
"This is still a rescue mission until we determine otherwise," Hots said. "We don't have a firm idea of how many people are out there."
Shari Ireton, spokesperson for the Snohomish County Sheriff's office, said rescuers were using thermal imaging cameras to help try to find people.
The slide blocked the North Fork of the Stillaguamish River, which prompted an evacuation notice because water was rising rapidly behind the debris. Authorities worried about severe downstream flooding if water suddenly broke through the blockage.
The landslide also completely covered State Route 530 near the town of Oso, about 55 miles north of Seattle. It was at least 135 feet wide and 180 feet deep and hit just before 11 a.m., Snohomish County authorities said.
The Snohomish County sheriff's office reported that two people had been killed at the scene. Authorities later said one of the people who was rescued died at a hospital.
The injured included a 6-month-old boy, who was in critical condition at Harborview Medical Center in Seattle. Hospital spokeswoman Susan Gregg said two other victims were in critical condition an 81-year-old man and a 37-year-old man while a 58-year-old man and a 25-year-old woman were in serious condition.
Five of the injured were brought to Cascade Valley Hospital in Arlington, and one has already been treated and released, said hospital spokeswoman Jennifer Egger. She didn't know the condition of the others.
The American Red Cross set up at the hospital, and evacuation shelters were created at Post Middle School in Arlington and the Darrington Community.
One eyewitness told the Daily Herald that he was driving on the roadway and had to quickly brake to avoid the mudslide.
"I just saw the darkness coming across the road. Everything was gone in three seconds," Paulo Falcao told the newspaper.
Search-and-rescue help came from around the region, plus the Washington State Patrol and the Army Corps of Engineers.
Hots said crews heard voices late Saturday night on the eastern edge of the debris field. Snohomish County Sheriff Ty Trenary said "we have rescuers on the ground on both sides of the slide who are going to be there all night, we're combing through the debris field on the ground trying to rescue people."
Air operations to aid rescuers were suspended, but authorities said they would resume at first light Sunday. Ireton also said the number of destroyed homes was expected to increase when crews had more time to assess the scene.
The National Weather Service issued a flash flood watch for Snohomish County through Sunday afternoon.
People who live in the North Fork's flood plain, from the small communities of Oso to Stanwood, were urged to flee to higher ground.
Forecasters warned that some flooding was also possible north of the slide area. The Weather Service said "catastrophic flooding" was unlikely downstream, but authorities were taking no chances and urged people to leave.
Washington Gov. Jay Inslee also proclaimed a state of emergency.
Bart Treece, spokesman for the Washington State Department of Transportation, said he didn't know how long the two-lane rural road will be closed because of the slide. Drivers were advised to find another way to get between Darrington and Arlington, he said.
Authorities believe the slide was caused by ground water saturation from recent heavy rainfall. John Pennington from the Snohomish County Department of Emergency Management said the area has a history of unstable land. He said a slide also happened there in 2006.
Pennington said the most recent incident happened without warning.
"This slide came out of nowhere," he said.