LoAnna Langton saw the mountain move.

It roared. It shook. It tossed aside trees — 100 at a time, she said — as it tumbled in an avalanche of mud and rock toward her back door. It moved so fast she immediately gave up any hope of surviving. Langton gathered the children and hugged them close, so they would be buried in the same spot. “I just knew,” she said, “we were going to die.”

But Washington state’s deadliest natural disaster since the 1980 Mount St. Helens volcanic eruption — and one of the worst in a century — came to a miraculous stop in Langton’s back yard. Elsewhere in her small rural town of Darrington, residents were killed in a variety of ways: Some were swept off a highway in their cars, others were crushed by huge mud balls that demolished everything in their path, and many simply disappeared.

At least 18 people have been confirmed dead, and up to 30 more are likely entombed in a thick gray muck, swallowed by the land, perhaps never to be seen again.

For two tiny logging towns — Darrington’s population is 1,400 and Oso’s is 180 — the death on such a wide scale is unimaginable. Bogged down by relentless rain that continued Saturday, workers have failed to unearth many bodies, all but assuring that the landscape will be preserved in the short term as a mass graveyard.

What made the mountain move

Nearly every adult in this part of Snohomish County, a patchwork of little towns about 50 miles north of Seattle, knew someone who died or knows someone close to them. It is a tightknit community of loggers who earn low-wages in jobs cutting timber, and wealthy people, some of them retirees, who built dream homes on emerald hills with scenic views of the Cascades, near a river teeming with steelhead and salmon.

But some are now questioning whether many of the homes should have been built at all in the valley below a hillside that commonly shifts, sending mud raining down about once a decade. At least four new homes have been built since the last major landslide muddied the valley eight years ago.

The Snohomish County officials who control land use permits asserted last week that there was no way of knowing a giant mudslide would ever happen there.

In fact, the area was primed for just such an extraordinary event, according to geologist Daniel J. Miller, who twice surveyed the area for local Native American tribes who rely on the river’s health for fishing and for the Army Corps of Engineers. He wrote in his 1999 report that the Hazel Landslide, as the mountain is known, was constantly shifting, experiencing landslides and would one day suffer “a catastrophic failure.”

“This landslide moves every year when it gets wet, and pieces fall off,” said Miller, a consultant in Seattle, in a telephone interview Friday.

It was a nightmare waiting to happen.

An ancient glacier is jutting out of the mountain, making its flat plateau unstable, Miller said. The Stillaguamish River was eroding it from below. Rows of conifer trees that helped to mitigate erosion by sucking water through their roots and releasing it into the atmosphere were chopped down by loggers. Rain fell on the bald spots they left, drenching dirt and sand, making the mountain even more precarious.

March 2014 has been a ­record-breaker, the wettest in Seattle’s history.

Miller realized his warning was not heeded when he visited the site following a major landslide in 2006 that did not do nearly as much harm. He could not believe what he saw.

“There was new construction,” he said. “The sound of hammering competed with the sound of [destabilized] trees snapping after the mudslide. I can’t believe that someone wanted to build their home there. It was a very bad idea.”

Charity Prueher, 41 and raised in Oso, said homeowners rarely mentioned the slides. When they did, the coursing mud was considered a small disruption, more of an annoyance than a major problem.

“They’re so content with the beautiful place where they live, they don’t think anything would happen,” Prueher said.

Prueher said she helped clear debris from the 2006 mudslide when she was a volunteer firefighter. The thought that another slide could come that was far worse never occurred to her.

“It was nothing compared to this,” she said. State geologists estimated the volume of the March 22 slide at about 7 million cubic yards, covering an area equal to 545 football fields and six feet deep in parts.

As Prueher bowled with friends Thursday at Rocket Alley Bar and Grill in Arlington, the largest town nearby, reality sank in. “I’ve been over this in my head many times in the last five days,” Prueher said. “I know close to 20 people who are missing and haven’t been found.”

That includes a family of four who own a store across from the school where Prueher works. When she thought of the family’s two little boys, laughing and playing outside in the spring, her eyes started welling with tears.

“I know I’m never going to hear them laughing again,” she said.

Then she caught herself. “You should always have hope,” she said. But her expression reflected the grim reality that has washed over this county. She feels deep down that all the missing are dead, but “I can’t bring myself to say it out loud.”

After a tragedy writ large, disbelief and grief often turn to anger and a demand for accountability. Already, some engineers have criticized local and state officials for failing to recognize the dangers of development on the mountain.

David Montgomery, a professor of geology at the University of Washington, said questions must be asked, but he is not sure blame is deserved.

Predicting mudslides is like forecasting the weather or an earthquake, he said. The science is not exact.

Montgomery praised Miller, a colleague, because his 1999 prediction appears prophetic in hindsight. But Miller had no way of knowing when his prediction would come true, and he did not take the added step of estimating where debris would flow when the slide happened.

A report such as that might have gotten the attention of policymakers, he said. And then again, it might not have.

Geologists have recorded landslides around Seattle for generations, but dire warnings about shaky ground in their reports are not always greeted as good news. It can hurt homebuilding and businesses that generate tax income.

Regardless of who is to blame, Montgomery said, the state and other stakeholders must do whatever is needed to better understand the geology of its mountains and hills. “We have to use that information to identify the true hazard areas and run-out zones,” where the cascading debris is likely to go.

When the mountain began to move on an ordinary Saturday morning across the Stillaguamish River, it sounded “like a 747 jet was directly overhead,” said Langton, 30.

She was feeding her baby when it started. Her kids playing outdoors yelled for her to come out. Right away, she knew something was weird.

“There was this gray cloud . . . and then I saw it taking out trees 100 at a time,” she said. “I started screaming for my kids to get out of there. I’m screaming, ‘It’s a mudslide!’ ”

Within minutes, the roar was gone, and Langton opened the back door.

“I looked out on complete devastation,” she said. “I heard two ladies shouting for help. I heard another voice screaming, ‘Help.’ I couldn’t see him. I said, ‘Sir, could you raise your arms so I can see you?’ He said he only had one arm. The other one was just hanging there.”

She helped rescue them. Now she lives in a basement of a friend’s home, because her house is flooded. Her husband, Kris, is among the volunteers slogging through the mud in the pelting rain and wind to find his neighbors. He rarely talks about what he sees.

“We’re a little logging community,” she said. “There are so many missing, so many dead. We definitely feel God protected us. My neighbor’s house is gone. My husband’s out there digging for bodies.”

Richard Johnson in Oso and Julie Tate and Alice Crites in Washington contributed to this report.