ARLINGTON, Wash. — Eighteen people remained unaccounted for Sunday, authorities say, a day after a massive mudslide killed at least three people and forced evacuations because of fears of flooding.
Washington Gov. Jay Inslee described the scene as "a square mile of total devastation" after flying over the disaster area Sunday. He assured families that everything was being done to find their missing loved ones.
"There is a full-scale, 100 percent aggressive rescue going on right now," he said.
Snohomish County Fire District 21 Chief Travis Hots said the number of missing is fluid and could change because some people may have been in cars and on roads when the slide hit just before 11 a.m. Saturday.
The mud was so thick and deep that searchers turned back late Saturday after attempting to reach an area where voices were heard crying for help.
Rescuers couldn't hear any signs of life once they got closer, and the decision was made to back out for safety reasons, Hots said.
Authorities say the slide wiped out one neighborhood of 28 to 30 homes. The mud was reported to be about 60 feet deep in some areas.
As authorities tried to determine how to get responders on the ground safely, helicopters were flying over the area looking for people who may have been able to get out on their own and for any other signs of life.
He said the slide debris has the consistency of quicksand and rescuers can't go back in until they get a better assessment of the conditions.
The slide has blocked the north fork of the Stillaguamish River.
"The mudflow is moving. We have a massive amount of water that is beginning to back up and has backed up since this thing blocked the river," Hots said.
People downstream from the blockage were advised to evacuate Saturday night, but the evacuation recommendation was lifted for daylight hours.
Emergency management director John Pennington said the flood potential makes the situation a "disaster within a disaster."
"We really have two disaster operations going — one that has occurred, and the unique thing about this event is that there is another event that most likely can occur," Pennington said.
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 flooding also was 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.
"Treat this as if you would any large flood warning in Snohomish County," Pennington said. "We're really good at this in this county. In this case it's a very serious issue."
The cause of the slide is believed to be groundwater saturation from heavy rainfall this month.
"It sounds very realistic with all the rain earlier in the month," AccuWeather senior meteorologist Alan Reppert said. "There hasn't been much precipitation in the last few days."
Everett, Wash., 31 miles southwest of Oso, has had 5.64 inches of rain this month -- more than 2-1/2 times what the city typically receives in March, AccuWeather reported.
Authorities are using helicopters and plan to drop rescuers into areas if they find signs of life.
First responders in Snohomish County, Wash., called it 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 that the river was blocked by the slide.
Contributing: Doyle Rice in McLean, Va.; William M. Welch in Los Angeles; The Associated Press