Pennington said the landslide risk has been high this winter, and the Department of Natural Resources put out warnings on a routine basis.
He added officials will try to learn from this tragedy.
Authorities also told reporters they expect to soon have an updated number of people believed missing.
They are working off a list of 176 unaccounted for, though some names were thought to be duplicates and the number should decrease. Pennington said officials would have a revised figure later Wednesday.
Two additional bodies were recovered Tuesday, while eight more were located in the debris field from Saturday's slide 55 miles northeast of Seattle. That brings the likely death toll to 24, though authorities are keeping the official toll at 16 until the eight other bodies are recovered.
"We haven't lost hope that there's a possibility that we can find somebody alive in some pocket area," said Snohomish County District 21 Fire Chief Travis Hots.
Authorities said they are doing everything they can to keep responders safe as the increasingly desperate search progresses in mud and debris amid the threat of flash flooding. Searchers "got beat up" in Tuesday's rainy weather, operations section chief Steve Westlake noted.
A 2010 report commissioned by Snohomish County to comply with a federal law warned that neighborhoods along the Stillaguamish River were among the highest-risk areas, The Seattle Times reported.
The hillside that collapsed Saturday outside of the community of Oso was one highlighted as particularly dangerous, according to the report by California-based engineering and architecture firm Tetra Tech.
"For someone to say that this plan did not warn that this was a risk is a falsity," said report author and Tetra Tech program manager Rob Flaner.
A 1999 report by geomorphologist Daniel Miller, although not about housing, raised questions about why residents were allowed to build homes in the area and whether officials took proper precautions.
"I knew it would fail catastrophically in a large-magnitude event," though not when it would happen, said Miller, who was hired by the U.S. Army Corps of Engineers to do the study.
A year later, the Army Corps warned in another study that lives would be at risk if the hillside collapsed, The Daily Herald of Everett reported.
Residents and county officials were focused on flood prevention, even after the 2006 landslide that did not reach any homes.
"We were just trying to stabilize the river so we could save the community from additional flooding," said Steve Thomsen, the county's public works director.
The area has long been known as the "Hazel Landslide" because of landslides over the past half-century.
Steven Swanson, 66, lived in the slide area for several years in the 1980s.
"I've been told by some of the old-timers that one of these days that hill was going to slide down," said Swanson, who now resides in Northport in northeast Washington. "County officials never said anything to me about it while I lived it there, just the old-timers who grew up there."
Predicting landslides is difficult, according to a study published by the U.S. Geological Survey in 2012. One challenge is estimating the probability of a slide in any particular place.
One of the authors, USGS research scientist Jonathan Godt in Colorado, said landslides don't get that much attention because they often happen in places where they don't hit anything.
Homeowners insurance typically does not cover landslide damage, but customers can buy such coverage, said Karl Newman, president of the NW Insurance Council, a trade group in the Northwest.
Pennington, the local emergency management official, choked up as he spoke of the help the region has received.
"It is very humbling. And we're respectfully, very grateful," he said.
Le reported from Seattle. Associated Press writers Jason Dearen in San Francisco; Lisa Baumann in Seattle; P. Solomon Banda in Darrington; and photographer Elaine Thompson in Oso contributed to this report. Researcher Rhonda Shafner contributed from New York.