Amazingly, nobody was killed. The three people who fell into the water escaped with only minor injuries.
Officials are trying to find out whether the spectacular collapse of a bridge on one of the West's most important roadways was a fluke — or a sign of a bigger problem with thousands of bridges across the U.S.
Authorities focused first on trying to find a temporary span for the Skagit, although it won't come in time for the tens of thousands of Memorial Day vacationers who would travel between Canada and Seattle.
"You cannot overstate the importance of this corridor to Washington state," Gov. Jay Inslee said. Traffic on I-5 and surrounding roads was backed up for miles, a situation the governor said would continue indefinitely.
Officials were looking for a temporary, pre-fabricated bridge to replace the 160-foot section that failed, Inslee said Friday. If one is found, it could be in place in weeks. If not, it could be months before a replacement can be built, the governor said.
The spectacular collapse unfolded about 7 p.m. Thursday on the north end of the four-lane bridge near Mount Vernon, about 60 miles north of Seattle and 40 miles south of the Canada border.
"He looked in the mirrors and it just dropped out of sight," Cynthia Scott, the wife of truck driver William Scott, said from the couple's home near Spruce Grove, Alberta. "I spoke to him seconds after it happened. He was just horrified."
The truck driver works for Mullen Trucking in Alberta, the Washington State Patrol said. The tractor-trailer was hauling a housing for drilling equipment southbound when the top right front corner of the load struck several of the bridge's trusses, the patrol said.
Scott, 41, remained at the scene and cooperated with investigators. He voluntarily gave a blood sample for an alcohol test and was not arrested.
Scott has been driving truck for 20 years and hauling specialized loads for more than 10.
"He gets safety awards, safety bonuses ... for doing all these checks, for hiring the right pilot cars and pole cars," his wife said.
Initially, it wasn't clear if the bridge just gave way on its own. But Washington State Patrol Chief John Batiste blamed it on the too-tall load. The vertical clearance from the roadway to the beam is 14.6 feet.
The truck made it off the bridge, but two other vehicles went into the water about 25 feet below as the structure crumbled.
Dan Sligh and his wife were in their pickup heading to a camping trip when he said 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.
Bryce Kenning, of Mount Vernon, said the bridge seemed to explode in front of him. The 20-year-old slammed the brakes and could see the edge of the pavement approaching, but there was nothing he could do.
"It was like time was frozen — like a roller coaster where you're not attached to the tracks," Kenning said in a phone interview. "I'm sure it was just one of the loudest sounds ever to hear this thing explode and fall into the water like that, but I didn't hear a thing. I just witnessed it happening in front of me."
Ed Scherbinski, vice president of Mullen Trucking, said in an interview with The Associated Press that state officials had approved of the company's plan to drive the oversize load along I-5 to Vancouver, Wash., and the company hired a local escort to help navigate the route.
Mike Allende, a state Department of Transportation spokesman, confirmed the truck had a permit.
"We're still trying to figure out why it hit the bridge," Allende said. "It's ultimately up to the trucking company to figure out whether it can get through."
State officials approved the trucking company to carry a load as high as 15 feet, 9 inches, according to the permit released by the state. However, the southbound vertical clearance on the Skagit River bridge is as little as 14 feet, 9 inches, state records show. The bridge's curved overhead girders are higher in the center of the bridge but sweep lower toward a driver's right side.
The bridge has a maximum clearance of about 17 feet, but there is no signage to indicate how to safely navigate the bridge with a tall load.
The permit specifically describes the route the truck would take, though it includes a qualification that the state "Does Not Guarantee Height Clearance."
It's not rare for trucks to strike bridges in Washington state — it's just that such accidents don't usually cause the structures to collapse.
The state DOT said there were 21 bridge-strikes involving trucks last year, 24 in 2011 and 14 in 2010.
Officials performed a special inspection six months ago of the bridge that collapsed because there were indications it had been struck by a different vehicle.
A report released Friday says the checkup was done due to "impact damage," and inspectors identified tears, deformations and gouges on the northbound side of the bridge. The report also summarizes a variety of parts on the bridge that have been subjected to "high-load" hits.
State officials did not respond to a phone call seeking more detail.
There are no signs leading up to the Skagit River bridge to warn about its clearance height. State Transportation Secretary Lynn Peterson said that under federal and state standards, the clearance is tall enough to not require signage.
Inslee said it will cost $15 million to repair the bridge. The federal government has already promised the state $1 million in emergency funding.
Traffic could be affected for some time. The bridge is used by an average of 71,000 vehicles a day, so the roadblock will cause a major disruption in trade and tourism.
The closest detour is a bridge about a quarter mile east of I-5, which is mostly used by local traffic between Mount Vernon and Burlington. Officials are also recommending detours using state Routes 20 and 9 that add dozens of miles to a trip.
A Federal Highway Administration database lists the bridge that collapsed as "functionally obsolete" — a category meaning that the design is outdated, such as having narrow shoulders and low clearance underneath. But it was not classified as structurally deficient.
The 1,112-foot-long bridge, with two lanes in each direction, has four spans, or sections, over the water supported by piers. It's a steel truss bridge, meaning it has a boxy steel frame.
The northernmost span is the one that collapsed.
The mishap was reminiscent of the August 2007 collapse of an I-35W bridge in Minneapolis that killed 13 people and injured another 145 when it buckled and fell into the Mississippi River during rush-hour.
The National Transportation Safety Board determined that the Minneapolis bridge failed because steel gusset plates that connected the structure's beams and girders were too thin.
Baker reported from Olympia, Wash. Associated Press writers Donna Gordon Blankinship and Gene Johnson in Seattle, and Terry Tang in Phoenix also contributed to this report.