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 are also 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.

“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. “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.

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

Bryce Kenning, of Mount Vernon, said the bridge seemed to explode in front of him. The 20-year-old slammed on his brakes, 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.