MOUNT VERNON, Wash. — Washington state officials are scrambling to find a temporary fix for a bridge that collapsed on an important interstate highway and, incredibly, left just three motorists with injuries. Whatever the solution, it won’t come in time to help with Memorial Day’s highway hoards.

Transportation experts also are working to find out whether the spectacular disintegration of the heavily used span over the Skagit River, 60 miles north of Seattle, was a fluke or a sign of a bigger problem.

A trucker was hauling a load of drilling equipment Thursday evening when his load bumped against the steel framework over an Interstate 5 bridge. He looked in his rearview mirror and saw the span collapse into the water behind him.

Two vehicles fell into the chilly river. The three people in those vehicles were not seriously hurt.

As authorities try to find a temporary span, motorists will have to use alternate routes during the holiday and for weeks to come.

“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, prefabricated bridge to replace the 160-foot section that failed, Inslee said Friday. That option could be in place in weeks. Otherwise, it could be months before a replacement can be built, the governor said.

The collapse came on the north end of the four-lane bridge near Mount Vernon, about 60 miles north of Seattle.

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.

The truck made it off the bridge, but the other two vehicles went into the water about 25 feet below.

Mike Allende, a state Department of Transportation spokesman, confirmed the truck had a permit.

Inslee said it will cost $15 million to repair the bridge. The federal government has promised $1 million.

The bridge is used by an average of 71,000 vehicles a day.