Still, he says one acre of harvest appears to have been removed within the restricted groundwater recharge area. Immediately following the slide, the department initiated a review of forestry-related activities in the area.

The Stillaguamish River cutting into the toe of the hillside could have played a role as well, Montgomery says.

Patricia Graesser, spokesperson for the Seattle District of the U.S. Army Corps of Engineers says her agency was working with Snohomish County to evaluate sites for restoration potential, which included the Hazel Slide site.

“What it documented for us was the history of material sloughing into the river and the likelihood of it continuing to slough into the river,” she says.

The Oso homes reside outside of a navigable waterway or wetland, so the Corps had no jurisdiction over construction permits there.

A 2010 Natural Hazard Mitigation Plan Update for the county that was written by the Seattle office of engineering firm Tetra Tech included an entire chapter on landslide danger.

It highlighted the Hazel Slide area near Oso as a dangerous location due to the steepness of the slope and unstable soil.

The slide has also closed down both directions of State Route 530 indefinitely, officials said.

When the area is safe enough, state transportation department crews will enter to evaluate highway damage and formulate a plan to clear debris and make repairs, said an agency statement.

Already the state DOT is requesting that potential contractors who could help with the road cleanup contact the agency.