The major highway bridge linking Seattle with Canada and the rest of the Pacific Northwest region has collapsed, dumping a handful of vehicles and people into a river, the Washington State Patrol said.
Authorities said there were no deaths, and three people were rescued and taken to hospitals.
The four-lane Interstate 5 bridge - more than half a century old - collapsed about halfway between Seattle and Vancouver, British Columbia, Trooper Mark Francis said.
Francis said he did not know what caused the collapse, which came at the start of one of the country's busiest holiday weekends of the year. The collapse came before sundown on a clear day.
The state Transportation Department said it was investigating whether an oversize truck load may have struck the bridge.
Dan Sligh and his wife were in their pickup truck heading to a camping trip when the bridge before them disappeared in a "big puff of dust."
"I hit the brakes and we went off," Sligh told reporters from a hospital, adding he "saw the water approaching ... you hold on as tight as you can."
Sligh, his wife and another man in a different vehicle were dumped into the chilly waters of the Skagit River.
"We don't think anyone else went into the water," said Marcus Deyerin, a spokesman for the Northwest Washington Incident Management Team.
"At this point we're optimistic."
It was not known what caused the collapse of the bridge about 100 kilometers north of Seattle, but State Patrol detectives and the patrol's commercial vehicle enforcement bureau troopers were talking Friday (NZT) to a commercial truck driver whose rig was believed to have struck the structure.
"It appears the commercial vehicle made contact with the bridge," Washington State Trooper Mark Francis said. "Whether it was the cause" of the collapse or made contact as the bridge was falling "that will all come out in the wash. But it appears it hit the bridge."
Sligh said his shoulder was dislocated in the drop into the water, and he found himself "belly deep in water in the truck." He said he popped his shoulder back in and called out to his wife, who he described as being in shock initially as they waited for rescuers to arrive in boats.
Traffic along the heavily travelled route could be affected for some time.
"The I-5 corridor is totally disrupted," said Gov. Jay Inslee, who went to the scene.
Jeremiah Thomas, a volunteer firefighter, said he was driving nearby when he glimpsed something out of the corner of his eye and turned to look.
"The bridge just went down, it crashed through the water," he said. "It was really surreal."
Deyerin said it appeared that two vehicles - a car and the pickup with the travel trailer attached - fell into the river. He said the water depth was about nearly 5 meters, and the vehicles half-visible in the water likely were resting on portions of the collapsed bridge.
Crowds of people lined the river to watch the scene unfold.
"It's not something you see every day," said Jimmy O'Connor, the owner of two local pizza restaurants who was driving on another bridge parallel to the one that collapsed. "People were starting to crawl out of their cars."
The bridge was not classified as structurally deficient, but a Federal Highway Administration database listed it as being "functionally obsolete" - a category meaning that the design is outdated, such as having narrow shoulders and low clearance underneath.
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, but 759 bridges in the state have a lower sufficiency score.
Democratic Rep. Judy Clibborn, who leads the transportation committee in the state House, said the bridge wasn't one that has been a focus for lawmakers.
"It is shocking that I-5 would have something happen like this," she said.
Clibborn said the collapse will call attention to the issues facing bridges - especially the old bridge over the Columbia River that connects Vancouver, Washington, and Portland, Oregon.
Sligh said his wife was "doing OK" and that he had "lots of cuts."
"You're kind of pinching yourself and realise you're lucky to be alive."