The driver, William Scott, of Spruce Grove, Alberta, near Edmonton, voluntarily gave a blood sample for an alcohol test and was not arrested.
Initially, it wasn't clear if the bridge just gave way on its own. But at an overnight news conference, Washington State Patrol Chief John Batiste blamed it 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 and the driver remained at the scene and cooperated with investigators. Two other vehicles went into the water about 25 feet below as the structure crumbled. Three people were rescued and were recovering Friday.
The trucking company said it received a state-issued permit to carry its oversized load across the bridge.
Ed Scherbinski, vice president of Mullen Trucking, said in an interview with The Associated Press that the Washington state Department of Transportation had approved of the company's plan to drive a piece of drilling equipment along Interstate 5 to Vancouver, Wash.
He also said the company hired a local escort to help navigate the route.
Scherbinski said company officials are as bewildered as everyone else. He said he's not sure whether the Mullen Trucking vehicle was the cause of the collapse, but the driver could see the bridge falling in his rearview mirror.
Cynthia Scott, of Spruce Grove, Alberta, said she spoke with her husband moments after he saw the bridge fall into a river in his rear-view mirror. Cynthia Scott said there was a small ding in one of the front corners of the load.
Dave Chesson, a state DOT spokesman, said there were no signs leading up to the bridge warning about its clearance height.
Traffic could be affected for some time. The bridge is used by an average of 71,000 vehicles a day, so the roadblock will cause a major disruption in trade and tourism between Seattle and Vancouver, British Columbia.
The Washington Transportation Department has set up detours. The closest bridge nearby is mostly used for local traffic between Mount Vernon and Burlington. The department also is recommending detours using Highway 20 and Highway 9 that add tens of miles to a trip. Drivers are urged to avoid the area if possible, especially over the Memorial Day weekend.
Francisco Rodriguez, of Burlington, looked at the damage Thursday evening and realized the area has lost an important transportation link.
"Well, very important, I mean everybody goes through here, everybody goes to Canada, Canadian side. Myself, I drove it every day, twice a day," he said.
Dan Sligh and his wife were in their pickup on Interstate 5 heading to a camping trip when a bridge before them disappeared in a "big puff of dust."
"I hit the brakes and we went off," Sligh told reporters from a hospital, adding he "saw the water approaching ... you hold on as tight as you can."
Sligh and his wife were taken to Skagit Valley Hospital with non-life-threatening injuries. The other man was reported in stable condition at United General Hospital in Sedro-Woolley, hospital CEO Greg Reed said.
The bridge was inspected twice last year and repairs were made, Transportation Secretary Lynn Peterson said.
"It's an older bridge that needs a lot of work just like a good number of bridges around the state," she said.
The National Transportation Safety Board was sending an investigative team.
Jeremiah Thomas, a volunteer firefighter, said he was driving nearby when he glimpsed something out of the corner of his eye and turned to look.
"The bridge just went down, it crashed through the water," he said. "It was really surreal."
Crowds of people lined the river to watch the scene unfold.
"It's not something you see every day," said Jimmy O'Connor, the owner of two local pizza restaurants who was driving on another bridge parallel to the one that collapsed. "People were starting to crawl out of their cars."
The bridge was not classified as structurally deficient, but a Federal Highway Administration database listed it as being "functionally obsolete" a category meaning that the design is outdated, such as having narrow shoulders and low clearance underneath.
The bridge, which was inspected last August and November, was built in in 1955 and had a sufficiency rating of 47 out of 100 at its November 2012 inspection, Transportation Department spokesman Noel Brady said Friday. The state average is 80, according to an Associated Press analysis.
Washington state was given a C in the American Society of Civil Engineers' 2013 infrastructure report card and a C- when it came to the state's bridges. The group said more than a quarter of Washington's 7,840 bridges are considered structurally deficient or functionally obsolete.
The bridge was 1,112 feet long and 180 feet wide, with two lanes in each direction, Brady said. There are four spans, or sections, over the water supported by piers. The span on the north side is the one that collapsed. It's a steel truss bridge, meaning it has a boxy steel frame.
The mishap was reminiscent of the August 2007 collapse of an I-35W bridge in Minneapolis that killed 13 people and injured another 145 when it buckled and fell into the Mississippi River during rush-hour.
Sligh was thankful.
His wife was "doing OK" and he had "lots of cuts," he said. "You're kind of pinching yourself and realize you're lucky to be alive."
Baker reported from Olympia, Wash. Associated Press writers Chris Grygiel in Seattle and Terry Tang in Phoenix also contributed to this report.