OSO — A new lawsuit filed over deaths and property destruction from last year’s catastrophic mudslide makes a startling claim.

It faults the state of Washington, Snohomish County and a private timber company for causing, “the worst man-made landslide in the history of this nation.”

Seattle attorney Karen Willie filed the suit on behalf of three families of people killed in the slide and others who lost property.

“I am certain that this is a man-made landslide — I don’t think it was an act of God,” Willie said.

The lawsuit doesn’t explain how that claim squares with the evidence geologists say exists of 15 other large slides in the valley over the past 6,000 years, occurring anywhere from 400 to 1,500 years apart. Slides on that particular stretch of hillside have been occurring at least back to the 1930s.

The March 22 disaster killed 43 people and destroyed about 40 houses in the Steelhead Haven neighborhood. The new Oso lawsuit is the third to date. Filed Jan. 21 in King County Superior Court, it seeks damages to be determined at trial.

Plaintiffs include the estates and relatives of John and Kris Regelbrugge; Lon Slauson; and Steve and Theresa Harris.

Three other sets of plaintiffs lost property: Henrietta Ottersen; Davis and Ruth Hargrave; and Irvin and Judith Wood.

The suit accuses the state Department of Natural Resources of negligence for issuing a logging permit in 2004 to Grandy Lake Forest Associates of Mount Vernon. The company’s 7.5-acre, pie-shaped clear-cut on top of the hillside played a major role in causing a 2006 slide that blocked the North Fork Stillaguamish River, the suit contends.

A DNR investigation concluded that permitting for the clear-cut was done properly, but that the company exceeded the approved size by an acre.

While there is less agreement on what caused the 2006 slide, recent scientific studies are in general agreement that it set the stage for catastrophe in 2014.

The suit accuses the state and Snohomish County of botching attempts to manage river flows through the area, including the use of a wooden crib wall at the toe of the slope. The state and county also failed to adequately investigate the 2006 slide or warn neighbors of the dangers, the suit argues.

Another prong in the lawsuit goes after the state for allowing what it describes as nuisance conditions on its property, which allowed soil and water to run onto the plaintiffs’ land.

Snohomish County was served with paperwork for the suit on Friday, chief civil deputy prosecutor Jason Cummings said. The county won’t comment on the pending litigation, but plans to put up a vigorous defense, Cummings said.

The suit draws from decades of scientific observations of 600-foot-tall Hazel hill, where the slide began.

“There were studies out there and people knew so much,” Willie said.

In 1947, a professor noted slumping blocks of soil on the hillside. A 1952 study suggested rerouting a stream called Headache Creek to improve the slope’s stability — at a cost then calculated at $5,400.

Steelhead Haven began getting built out in the 1960s, first as rudimentary fishing shacks.

A large landslide destroyed vacation cabins in the area in 1967. More studies followed.

Over time, more people began to settle the area as full-time residents.

In 1999, a now oft-cited study warned of a 900-foot runout — similar to what happened in 1967.

The damage last year — from the top of the scarp to its southernmost point — stretched 5,827 feet. That was the 10th slide to hit the same location, according to the lawsuit.

Of the two earlier suits over Oso deaths, the one representing the largest bloc of victims — 10 families — was filed in October. Like the current lawsuit, it faulted the state, the county and the same timber company.

The first lawsuit over the slide was filed in July.

Noah Haglund: 425-339-3465; nhaglund@heraldnet.com. Twitter: @NWhaglund.