SEATTLE -- A massive landslide of dirt, trees and rocks in rural Washington killed three people on Saturday, critically injured an infant and several others, and destroyed six houses, authorities said.
The slide that was at least 135 feet wide and 180 feet deep hit just before 11 a.m., Snohomish County authorities said.
The landslide completely blocked State Route 530 near the town of Oso, about 55 miles north of Seattle.
Also blocked was the North Fork of the Stillaguamish River, which prompted an evacuation notice because of concerns about possible severe downstream flooding affecting more homes.
People who live in the North Fork's flood plain, from the small communities of Oso to Stanwood, were urged to flee to higher ground.
Initially, the Snohomish County sheriff's office reported that two people had been killed in the landslide. Authorities said later that one of the people who was rescued died at a hospital.
The injured included a 6-month-old boy, who was in critical condition at Harborview Medical Center in Seattle.
Hospital spokeswoman Susan Gregg said two other victims were in critical condition -- an 81-year-old man and a 58-year-old man -- while a 37-year-old man was in serious condition. Gregg said Harborview had one other male patient from the landslide, but no information on his condition was available.
Five of the injured were brought to Cascade Valley Hospital in Arlington, but one has already been treated and released, said hospital spokeswoman Jennifer Egger.
The facility was expecting more injured people, but Egger said she didn't know how many and couldn't comment on the condition of those already at the hospital because they were still being evaluated.
"We're on standby waiting to see what happens next," Egger said.
The American Red Cross has set up at the hospital and is seeking donations of food, water, blankets and clothing, Egger said. An evacuation shelter has been set up at Post Middle School in Arlington.
One eyewitness told the Daily Herald that he was driving on the roadway and had to quickly brake to avoid the mudslide.
"I just saw the darkness coming across the road. Everything was gone in three seconds," Paulo Falcao told the newspaper.
Search-and-rescue help came from around the region, plus the Washington State Patrol, and the Army Corps of Engineers.
Authorities believe the slide was caused by ground water saturation from recent heavy rainfall.
The National Weather Service has issued a flash flood watch for Snohomish County through Sunday afternoon.
If the blockage in the river gives way, flooding downstream toward the larger community of Arlington was possible, forecasters said.
Spokesman Bart Treece of the Washington State Department of Transportation said he didn't know how long the two-lane rural road will be closed. Drivers were advised to find another way to get between Darrington and Arlington, he said.
"We're standing by ready to help out where we can," Treece said.