On Monday evening, President Barack Obama signed an emergency declaration and ordered federal aid added to state and local efforts, officials said at a press conference. This allows the Federal Emergency Management Agency (FEMA) to coordinate in the search and rescue efforts.
Snohomish County's emergency management director, John Pennington, said that the number of reports of missing people was raised to 176 on Monday. The list was pulled together from various sources that authorities are working from, and it does not necessarily mean there are that many injuries or fatalities.
But Pennington cautioned against holding out hope for many more survivors.
"Most of us here in the community believe we will not find anyone else alive," he said. "I'm a man of faith ... So does that mean we give up? No. But we are moving towards the realization that we're moving towards a recovery operation."
Among the possibly missing are construction workers commuting to the area and people just driving through.
Authorities said Monday they are no longer hearing people crying for help in the wreckage of the mudslide, dashing hopes of finding more survivors.
Search and rescue teams took to the air in helicopters and the ground on foot on Sunday to look for anyone who might still be alive. Their spirits were raised late Saturday night when they heard cries for help from the tangle of trees, dirt and destroyed houses. Dangerous conditions forced them to turn back in the darkness.
Adding to the worries was that the slide struck Saturday morning, when most people were at home. Of the 49 structures in the neighborhood hit by the slide, authorities believe at least 25 were regularly occupied.
Crews were able to get to the soupy, tree-strewn mud, which was 15 feet deep in places, Sunday after geologists flew over the area in a helicopter and determined it was safe enough for emergency responders and technical rescue personnel to search for survivors, Hots said.
He added that they did not search the entire debris field, only drier areas safe to traverse.
Frequent, heavy rainfall and geography make the area prone to landslides. Less than a decade ago, another slide hit not far from Oso. Geologists and other experts said that the Stillaguamish River likely caused some erosion in the area, which was carved by glaciers millennia ago, and that recent heavy rainfall made ground unstable, leading to Saturday's slide.
Snohomish County estimated in a recent report that 28,500 people lived in parts of the county that are susceptible to slides.
Washington Gov. Jay Inslee described the scene as "a square mile of total devastation" after flying over the area midday Sunday. He assured families that everything was being done to find their missing loved ones.
The slide blocked the north fork of the Stillaguamish River, raising fears on Saturday of downstream flooding. But water began to seep through the blockage Sunday, alleviating some concerns.
Bruce Blacker, who lives just west of the slide, doesn't know the whereabouts of six neighbors. "It's a very close-knit community," he said as he waited at an Arlington roadblock before troopers let him through.
Search-and-rescue help came from around the region, including the Washington State Patrol and the Army Corps of Engineers.
Dane Williams, 30, who lives a few miles from the mudslide, spent Saturday night at a Red Cross shelter at the Arlington school. He said he saw a few "pretty distraught" people at the shelter who didn't know the fate of loved ones who live in the disaster zone.
"It makes me want to cry," Williams said.
Al Jazeera and wire services