OSO, Snohomish County — Searchers looking for victims of the massive mudslide in eastern Snohomish County reported hearing voices — possibly from children — coming late Saturday from an area hit by the slide.

“We have people who are yelling for help and we are out there,” Travis Hots, Snohomish County district fire chief, said about 10:30 p.m. “This is a massive slide, and we are in a very, very fluid and unstable situation ... This is still a rescue mission.”

With three people known dead, at least six homes destroyed and a state highway severed by the mudslide, officials advised people downstream of the area to evacuate, fearing that further damage could occur.

Riverside residents between the slide area and Arlington, 15 miles to the west, were advised to leave their homes for the night, because of the danger that the North Fork of the Stillaguamish River could burst through the blockage created by the slide and cause immediate, severe flooding.

John Pennington, emergency-management director for Snohomish County, said the threat of a flood was building through the evening, with water behind the blockage rising 10 to 12 inches every half-hour.

Although Snohomish County officials could confirm only six homes destroyed, they said that number was expected to rise.

Pennington said the area has been prone to slides in the past.

About a dozen people were injured in the 11 a.m. slide as a wall of mud crashed through homes, and rescuers continued searching for victims into the night.

The power, speed and severity of the slide were spectacular, as it swept over a 360-yard-long section of roadway with mud and debris up to 20 feet deep.

“In three seconds, everything got washed away,” said Paulo de Oliveira of Lynnwood, who was driving on Highway 530. “Darkness covering the whole roadway and one house right in the middle of the street.”

De Oliveira said he was behind two other vehicles when the slide hit.

“I came within about 50 feet of being washed out.”

He got out of his car and heard a woman scream from one of the swamped houses.

“Along the river, I saw one place where there were two homes and they were just gone. Nothing left but a portable toilet ... destruction all around.”

Saturday evening, state highway crews were prepared to close bridges downstream — even one over Interstate 5 — when water surges through the blockage.

“A river has got a lot of energy in it. When you’ve got a plug, the pressure is going to build up and it’s going to blow,” said Bart Treece, of the Washington State Department of Transportation (WSDOT).

Because a rush of water could erode bridge supports, WSDOT doesn’t want anyone on the spans until after the torrent has passed and any damage been inspected, he said. One of two I-5 spans over the river was built in the 1930s and is of particular concern, Treece said.

The slide hit about four miles east of the town of Oso, said Lt. Rodney Rochon, head of the Snohomish County sheriff’s special operations unit. Authorities were unsure exactly how many homes were damaged and were searching for additional victims through the day.

Rochon said a 6-month-old baby was airlifted to Seattle’s Harborview Medical Center. Hospital spokeswoman Susan Gregg said the baby was in critical condition.

Four other victims were also in Harborview: a 68-year-old man in critical condition; an 81-year-old man in critical condition; a 37-year-old man in serious condition; and another man, age and condition unknown, Gregg said.

Cascade Valley Hospital in Arlington received six victims; three were treated and released and three were still at the hospital Saturday night.

Skagit Valley Hospital in Mount Vernon reported that it was treating a 68-year-old man in serious condition.

The mudslide sent tons of debris into the river, causing the river to overflow its banks.

Robin Youngblood was sitting in the living room with her friend, Jetty Dooper, when they heard a crack.

“All of a sudden there was a wall of mud” about 25 feet high, she said. “Then it hit and we were rolling. The house was in sticks. We were buried under things, and we dug ourselves out.”

Youngblood said she scrambled onto the top of the clothes dryer, and Dooper onto a dishwasher.

Covered in mud and shivering, they waited for perhaps an hour until they were lifted a short distance by helicopter and placed on an ambulance, Youngblood said.

Fearing a potential for flooding, officials asked residents near the slide to evacuate their homes. An emergency shelter was opened at Post Middle School in Arlington.

Officials urged people to stay away from the area Saturday afternoon as the North Fork of the Stillaguamish River threatened to break through or spread around the debris dam, potentially causing further destruction.

“While we are not issuing an evacuation order at this time, we need residents upstream and downstream of the slide to prepare to leave their homes at a moment’s notice,” said Bronlea Mishler, deputy director of communications for Snohomish County.

“We are asking residents to prepare their homes, pets, livestock, etc., for immediate evacuation if and when it becomes necessary.”

A river gauge immediately downstream of the mudslide showed the water level had dropped from 3.1 feet to .9 feet an hour after the landslide occurred. By 3:30 p.m., it had dropped to .25 feet.

More than 100 rescuers searched inside the destroyed houses. One of the search teams had to be rescued after it got stuck in the mud.

Two hovercrafts were brought to the area for rescue efforts.

David Logan, 58, of Seattle, was among those waiting near the barricade on Highway 530. He said his brother lives in the slide area and he hadn’t heard from him.

“I know his house is destroyed,” Logan said.

Rochon said rescue operations and a broader search will need to be completed before heavier equipment can be brought in.

Red Cross officials converged on Post Middle School in Arlington, where they set up a shelter for victims of the slide and evacuated residents.

Red Cross disaster-relief coordinator Andy Hamack said the best thing members of the community can do is donate money.

Snohomish County has been saturated with rain this month, establishing the kind of unstable terrain that can lead to mudslides, said Johnny Burg, a meteorologist with the National Weather Service.

Seattle Times staff reporters Mike Carter, Paige Cornwell, Mike Lindblom and Ken Armstrong and news researcher Miyoko Wolf contributed to this report.

Four weeks for 99 cents of unlimited digital access to The Seattle Times. Try it now!