U.S. National Transportation Safety Board investigators were on the scene of the collapse, which occurred on the four-lane Interstate 5, the principal highway between Seattle and Vancouver, Canada.

The NTSB said its members would inspect the 1,111-foot (339-meter) Warren Truss bridge, which links the towns of Mount Vernon and Burlington, including its substructure, deck and superstructure. Underwater inspections would also be conducted, the agency said.

The truck driver, identified as 41-year-old William Scott, made it across the bridge and pulled his vehicle to the side of the road after the accident. Behind him, a married couple in a pick-up fell into the water, as did a man in a Subaru who then sat on the top of his submerged car, authorities said.

Truckers must obtain a permit from the state Department of Transportation to carry an oversize load. The driver also receives a proposed route from the state, Washington State Patrol spokesman Sergeant Kirk Rudeen said.

An official with Canada-based Mullen Trucking, which employed the truck driver, said state transportation officials had given clearance to take the oversize loan across the bridge.

“Hopefully we will get some answers,” said Ed Scherbinski, vice president of operations for Mullen Trucking, adding that the company was sending its own investigative team to the scene.

The driver, Scott, has not been arrested and was cooperating with authorities, Rudeen said.

Scherbinski declined to immediately provide the height of the oversize load the trucker was hauling. The truck had been bound for Vancouver, Washington, he said.