The major highway bridge linking Seattle with Canada and the rest of the Pacific Northwest region collapsed, dumping vehicles and people into a river.
Authorities said there were no deaths, and three people were rescued and taken to hospital.
The four-lane Interstate 5 bridge – more than half a century old – collapsed about halfway between Seattle and Vancouver, British Columbia, Washington State Patrol trooper Mark Francis said.
The collapse came at the start of one of the country’s busiest holiday weekends of the year and before sundown on a clear day.
It will raise questions about the nation’s infrastructure, which has been a popular issue with President Barack Obama, who earlier this year warned against “raggedy” roads and wants to focus more money on rebuilding to improve the economy.
The American Society of Civil Engineers’ 2013 infrastructure report card gave the country a dismal grade of D-plus overall.
Xavier Grospe, who lives near the river, said he could see three partially-submerged cars and the apparent drivers were sitting either on top of the vehicles or on the edge of open windows.
“It doesn’t look like anybody’s in danger right now,” he said.
Helicopter footage on KOMO-TV in Seattle showed one rescue boat leaving the scene with one person strapped into a stretcher. A damaged red car and a damaged pick-up truck were visible in the water, which appeared so shallow it barely reached the top of the car’s bonnet.
Kari Ranten, a spokeswoman for Skagit Valley Hospital, said two people who were injured in the collapse were being taken there and another person was being taken to a different hospital.
A search of the river continued with a dive team was on the scene.
A man told the local Skagit Valley Herald newspaper he felt a vibration and looked in his rearview mirror to see that the part of bridge he had just crossed was no longer behind him. “I thought something was wrong with my car at first,” he said.
The bridge that collapsed is not considered structurally deficient but is listed as being “functionally obsolete” – meaning that its design is outdated, according to a database compiled by the Federal Highway Administration.
The bridge was built in 1955 and has a sufficiency rating of 57.4 out of 100, according to federal records. That is well below the statewide average rating of 80, according to an Associated Press analysis of federal data.
The American Society of Civil Engineers’ latest infrastructure report card said more than a quarter of Washington state’s 7,840 bridges were considered structurally deficient or functionally obsolete.