The landslide heaved houses off their foundations, toppled trees and left a gaping cavity on what had been a tree-covered hillside.
More than 100 properties were hit by the mudslide.
Seattle Times newspaper reported that many warnings had been issued about the area where the disaster.
In 1999, a report was filed to the U.S. Army Corps of Engineers highlighting “the potential for a large catastrophic failure”.
Search crew workers were forced again to briefly retreat on Monday from the western edge of the slide area after movement was detected along a 1,500-foot (460-meter) stretch of earth.
There is fear of flooding as water levels rose behind a crude dam of mud and rubble that had been dumped into the North Fork of the Stillaguamish River by the slide in an area along State Route 530, about 55 miles northeast of Seattle, in the Pacific Northwest state.