Three people died Saturday after a huge landslide came down on homes near Oso.
Snohomish County Fire District 21 Chief Travis Hots said at a news briefing late Saturday that people were "yelling for our help" and rescuers would work through the night.
Hots said it wasn't clear how many people might still be alive.
The massive slide of mud, trees and rocks happened about 11 a.m. Saturday morning.
At least eight people, including a six-month-old baby, were taken to a local hospital.
The slide cut off the city of Darrington and dammed up the North Fork of the Stillaguamish River, causing the water to pool behind the dam.
First responders in Snohomish County called it the worst slide, the worst natural disaster, they've seen in decades.
"Think back to what Mount St. Helens and Toutle River looked like -- and that's what we're looking at," said Rodney Rochon of the Snohomish County Sheriff's Office.
The National Weather Service said the river level dropped from 3.1 feet to 0.9 feet in the hour after the slide occurred, confirming the river was blocked by the slide.
The state hydrologist measured 15-20 feet of debris blocking the river, said KING 5 Meteorologist Mary Lee.
Since the landslide the river flow is 1,000 cubic feet per second, the National Weather Service said. For flooding to occur it would need to be 36,000 cubic feet per second.
For a catastrophic flood event to occur there would need to be a complete blockage for 36 hours with water suddenly released within an hour's time.
Snohomish County officials advised residents downstream of the slide to evacuate their homes.
"It is going to break loose and the question is how and where," said John Pennington with the Department of Emergency Mgt.
Shelters have been set up at Post Middle School in Arlington and the Darrington Cummunity Center. The Red Cross is assisting.
Officials warned people to stay clear of all trestles and bridges on the north fork of the river.
The landslide was first reported around 10:45 a.m. at the 29400 block of SR 530, near milepost 37.
Paulo Falcao told KING 5 the slide came down right in front of him.
"I was coming down the hill and I just saw darkness, like somebody wants to grab you, and everything was gone," he said.
More than 100 first responders converged on the scene, including those from Washington State Patrol, Department of Transportation, Department of Emergency Management, US Navy, Snohomish County Sheriff’s Office and most all of the fire department agencies in north Snohomish County.
If you are directly impacted by the slide you can register on the Red Cross website safeandwell.org so family and friends will know you are OK.
Follow @snocounty and @SnoCoSheriff for the latest updates on the situation.