The collapse happened at about 7 p.m. Thursday on the north section of the four-lane Interstate 5 bridge near Mount Vernon, about 60 miles north of Seattle and 40 miles south of the Canada border.
Authorities believe a semi-truck hauling an oversized load of drilling equipment hit an overhead bridge girder, sending a section of the span into a river below. Two vehicles plunged 40 feet into the Skagit River, but all three occupants escaped with only minor injuries.
"We're looking at the cause being an oversized, over-height vehicle, striking critical portions of this bridge, causing it to collapse," said Travis Phelps of the Washington State Department of Transportation and Washington State Patrol.
The driver, William Scott, of Spruce Grove, Alberta, voluntarily gave a blood sample for an alcohol test and was not arrested. He is cooperating with investigators.
The truck driver works for Mullen Trucking in Alberta. The trucking company told the Associated Press that it received a state-issued permit to carry its oversized load across the bridge, and that the Washington state Department of Transportation had approved of the company's plan to drive the equipment along Interstate 5 to Vancouver, Wash.
In addition, they had even hired a local escort to help navigate the route. He said the driver was well-experienced with handling oversized loads, said Ed Scherbinski, vice president of Mullen Trucking.
"This is what we do for a living. We pride ourselves in doing things the proper way," Scherbinski said.
Mike Allende, a state DOT spokesman, confirmed the truck had its permit.
"We're still trying to figure out why it hit the bridge," he said. "It's ultimately up to the trucking company to figure out whether it can get through. It's their responsibility to make sure the load they have can travel on that route."
Dave Chesson, a state DOT spokesman, said there were no signs leading up to the bridge warning about its clearance height.
The bridge, built in 1955, was not considered structurally deficient but was listed as "functionally obsolete" - a category indicating an outdated design, such as having narrow shoulders and low clearance underneath, according to a database compiled by the Federal Highway Administration.
Gov. Jay Inslee issued an emergency proclamation for surrounding counties Skagit, Snohomish and Whatcom. The governor believes it will cost $15 million to repair the Skagit River bridge. The federal government has already promised the state $1 million in emergency dollars to fix the Interstate 5 bridge.
The National Transportation Safety Board will be investigating the incident.
ABC News and The Associated Press contributed to this report.