The slide of mud, trees and rocks happened on 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 that searchers weren't giving up on finding more people.
"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 alive, he said.
The Snohomish County sheriff's office reported that two people had been killed at the scene. Authorities later said one person 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 58-year-old man — while a 37-year-old man was in serious condition.
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.