MOUNT VERNON, Wash. - The trucker was hauling a load of drilling equipment when his load bumped against the steel framework over an Interstate 5 bridge. He looked in his rearview mirror and watched in horror as the span collapsed into the water behind him. Two vehicles fell into the icy Skagit River.

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 collapse of a bridge on one of the West's most important roadways was a fluke - or a sign of a problem with thousands of bridges across the United States.

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

Officials were looking for a temporary, prefabricated 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 collapse occurred Thursday evening on the north end of the four-lane bridge near Mount Vernon, about 60 miles north of Seattle and 40 miles south of Canada.

"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. He has been driving a truck for 20 years and hauling specialized loads for more than 10.

Washington State Patrol Chief John Batiste blamed the collapse 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 fell about 25 feet into the water as the structure crumbled.

Ed Scherbinski, vice president of Mullen Trucking, said 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.