OLYMPIA, Wash. — Eighteen people remain unaccounted for in a landslide that killed three and injured at least eight more in northwestern Washington state, officials said Sunday.

The landslide destroyed at least six homes Saturday as it tumbled across a state highway, leaving a layer of mud, rock and debris 15 to 20 feet deep in some places, said Shari Ireton, spokeswoman for the Snohomish County sheriff’s office.

Ireton said it was unclear if some people still missing are trapped in the slide or have simply not reported their whereabouts. The sheriff’s office has asked people affected by the slide to report to the Red Cross so an accurate count can be made of the missing.

The slide happened as rain-soaked embankments along State Route 530 near Oso, about 55 miles northeast of Seattle close to the Canadian border, gave way at about 10:40 a.m. Saturday.

“Ground rescue operations remain extremely hazardous due to the debris field, which has been described by rescuers as a ‘quicksand-like consistency’,” Ireton said in a news release.

Voices were heard calling out Saturday evening, but officials determined it was too dangerous to send rescue workers in, said Snohomish County Executive John Lovick at a Sunday afternoon news conference in a nearby community.

The sound of voices had not been heard since Saturday, he said.

Washington Gov. Jay Inslee flew over the area Sunday morning before joining Lovick and other public officials at the news conference.

“The devastation is just unrelenting and awesome,” Inslee said. “There is really not a stick standing in the path of the slide.”

Inslee said the slide stretched across about a mile of the highway and posed a danger to rescue workers, including some who got mired in mud up to their armpits and had to be dragged to safety.

The slide occurred in the foothills of the Cascade Mountains along the Stillaguamish River. It has blocked flow of the river, creating floods and a backup of water behind a natural dam of mud and debris that could give way, Ireton said.

“We have a serious potential situation should that be breached,” Ireton said in an interview.

Helicopters, fire rescue vehicles and other equipment are being used to search for and remove any victims still in the rubble, officials said. The highway was closed in both directions, with no timeline for it to be reopened, Inslee said.

Washington Lt. Gov. Brad Owen on Saturday evening declared a state of emergency in Snohomish County.

State officials were providing information to federal authorities ahead of a possible federal declaration of emergency, said Inslee, who stressed that the rescue effort at present has all the resources it needs.