A large rescue operation began soon after the landslide struck on Saturday
The landslide destroyed six houses and covered a road near the town of Oso, about 90km (55 miles) north of Seattle.
It was at least 41m (135ft) wide and 54m deep, local authorities said.
They say that two people were killed at the scene and one died in hospital. The injured included a six-month-old boy.
An eyewitness told the Daily Herald that he was driving on the road 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.
The landslide cut off the city of Darrington and clogged the North Fork of the Stillaguamish River, local media reported.
Authorities say the landslide was caused by recent heavy rain.
More rain is expected in parts of Washington on Sunday, and some people have been advised to move to higher ground.
© BBC News
I know the area. People were buried alive. This was not a little slide, half the mountain came down. It wiped out a whole community and destroyed or damaged up to 30 houses. The Stilly is a very big, very mean river and right now is swollen from heavy rain and is rising behind its new earthen dam about a foot per hour. The large town of Arlington is several miles downstream and they are trying to get people to evacuate both above and below the blockage. They are used to having landslides up there, but This is different and a state of emergency has been declared. For more info check out KING 5 TV or KOMO TV 4.