SEATTLE — Dan Sligh was on his way to a camping trip with his wife and Navy colleagues when he noticed the oversized truck in front of him on Interstate 5 seemed about four feet too wide on the right side for the bridge they were approaching near Mount Vernon, Wash.

“I kept saying, ‘Anytime you want to move over to the left, it’d be OK,' ” Sligh told reporters. But just then, he said, another truck came up on the left of the oversize vehicle and the wide-load hit the approach spans to the four-lane bridge.

“There was a big puff of dust, and I hit the brakes. The weight of the trailer and everything else, [we] went right off the bridge as it collapsed into the Skagit River,” Sligh said.

PHOTOS: Bridge collapse in Washington

“The forward momentum just carried us right over, and as you saw the water approaching, it was just one of those things, you hold on as tight as you can,” he said, describing “a white flash, and cold water.”

The collapse of the northern span of the four-lane, 1,111-foot steel-and-concrete bridge shortly after 7 p.m. Thursday sent two vehicles and a trailer into the ice-cold river, triggering an urgent rescue effort as Sligh, his wife and another driver clung to their half-submerged vehicles.

All three were transported to hospitals and two were subsequently released, while Sligh’s wife, Sally Sligh, remained hospitalized Friday morning in stable condition.

Washington’s main north-south thoroughfare, though, was likely to remain closed 60 miles north of Seattle for an indefinite period, state officials warned. The nearly 71,000 vehicles a day that travel the bridge between Mt. Vernon and Burlington were diverted through city streets to another nearby bridge.

“I’m going to make sure this bridge is replaced as quickly as humanly possible. We’ll find a way to do this,” Gov. Jay Inslee told reporters.

Lynn Peterson, the state Transportation secretary, said a determination would be made as quickly as possible on whether to replace the entire bridge, or merely the segment that plunged into the river in a mass of twisted steel.

“We have to have some time to make sure we understand what damage was done,” she said.

Washington State Patrol Chief John Batiste confirmed that an oversize semi-trailer truck traveling southbound stuck an overhead beam on the steel-truss bridge, triggering a failure.

“The size of the load he was carrying appeared to create a problem, causing him to strike the bridge,” Batiste told reporters.

“We do have the truck driver who remained at the scene. We’ve had initial conversations with him to get an indication as to what occurred,” he said. The state patrol was encouraging anyone who witnessed the incident to phone in with reports.

The National Transportation Safety Board also was launching an investigation. It was not known whether the condition of the bridge played a role in the mishap.

The condition of the nation's aging bridge infrastructure has been an issue of concern since 2007, when a bridge fell into the Mississippi River in Minnesota, killing 13 people.

PHOTOS: 2007 Minnesota bridge collapse

The 58-year-old bridge in Washington, a crucial link to the Canadian border traveled heavily by trucks, was inspected every two years, most recently in November, state Department of Transportation spokesman Bart Treece told the Los Angeles Times.

“It’s an old bridge. We have to look into the specifics. We do have a lot of old, aging structures, and a lot of them hold up really well,” he said.

The National Bridge Inventory lists the bridge as “functionally obsolete,” with “somewhat better than minimum adequacy to tolerate being left in place as is.” It received a sufficiency rating of 57.4 out of 100.