Around 18 people are still missing after a massive mudslide in Washington State that killed at least four people and wiped out as many as 30 homes Saturday.

Searchers in helicopters flew over the mudslide near the town of Oso Sunday to find people who may have been able to get out on their own, and look for other signs of life.

Snohomish County Fire District 21 Chief Travis Hots said at a news briefing "we suspect that people are out there, but it's far too dangerous to get responders out there on that mudflow." Hots said on-scene crews reported hearing sounds of people calling for help Saturday night but since rescuers couldn't hear signs of life once they got closer, they decided to back out for safety reasons.

The National Weather Service issued a flash flood watch for Snohomish County through Sunday afternoon and Washington Gov. Jay Inslee also proclaimed a state of emergency. After flying over the scene Sunday, Inslee described the scene as “total devastation”.

The slide blocked State Route 530 near the town of Oso, about 55 miles north of Seattle as well as the North Fork of the Stillaguamish River. This prompted an evacuation notice as water was rising rapidly about 10 to 12 inches every 30 minutes Saturday night, according to John Pennington from the Snohomish County Department of Emergency Management.

Pennington said that emergency crews have the advantage of knowing that a potential flood could occur and are prepared.

“The unique thing about this event is that there is another that most likely can occur, a disaster within a disaster,” he said. “We have a glass-half full here in that we can know that a disaster might come downstream. We can see that, and we can act upon that.”

Reverse 9-1-1 calls went out to residents in the area warning them of flooding upstream from the slide as well as downstream should the river breach. On Sunday they were told they could return home in daylight hours but officials said they will likely reissue an evacuation Sunday night.

A 6-month-old boy and an 81-year-old man were in critical condition Sunday morning at Harborview Medical Center in Seattle. Hospital spokeswoman Susan Gregg said two men, ages 37 and 58, were in serious condition, while a 25-year-old woman was upgraded to satisfactory condition.

Hots was at the scene for much of Saturday afternoon and said there was debris as far as he could see.

“I was in awe,” he said. “It reminded me of the photographs that you would see and the video that we saw when Mt. St. Helens erupt[ed]. Trees laying over on their side. Massive mud debris.”

The affected area, which measured 6000 feet by 6000 feet or the equivalent of several neighbourhoods, was blocked by “nothing but mud”. The slide of dirt, trees, rocks and other debris was at least 135 feet wide and 180 feet deep. Police said the conditions were so treacherous that one of the search teams had to be rescued.

Paul Falcao witnessed the tragic slide from his vehicle.

“I was the third car behind a truck with a boat, and that’s when I just saw the darkness washing everything off the road,” he said. “I’m not sure if the truck made it through, but it didn’t look like [it].”

According to Colin Downey, communications director of the American Red Cross, two shelters were set up Saturday night and three additional Red Cross shelters were put on standby. Downey said that in situations like this, cash donations are most beneficial.

“If people do want to help, the best way to do that is through monetary donations through,” he said.

After surveying the scene, engineers will make recommendations on how to proceed with the clean-up.

Authorities believe the slide was caused by ground water saturation from recent heavy rainfall. Pennington said the area has a history of unstable land and Saturday’s slide “came out of nowhere”. He said a slide also happened there in 2006.

With files from the Associated Press