The sheriff said the identities of the victims would be confirmed by a medical examiner.
Washington State Patrol said earlier that the house has people inside it when the mudslide hit, just before 11 a.m. PT Saturday.
The Washington Department of Transportation says mud, trees and building materials are blocking both directions of State Route 530 near the town of Oso, about 20 kilometres east of Arlington.
Washington State Trooper Mark Francis said the mudslide is estimated to be a mile (1.6 kilometres) long.
Search and rescue operations were underway by Snohomish County crews and the Washington State Patrol.
Spokesman Bart Treece of the Washington State Department of Transportation says he doesn't know how long the two-lane rural road will be closed. He says drivers are advised to find another way to get between Darrington and Arlington.
"We're standing by ready to help out where we can," Treece said.