Washington State police say an Alberta trucker was responsible for hitting a steel beam precipitating a bridge collapse on one of the busiest routes in the American northwest.

William Scott, 41, was driving an oversized load on the Interstate 5 bridge over the Skagit River when it struck the beam Thursday, said State Patrol Sgt. Kirk Rudeen.

Scott's wife told CBC News that while his load clipped the bridge, he cannot be held responsible for the collapse.

"There was some speculation," said Cynthia Scott, from the couple's home in Spruce Grove, just west of Edmonton.

"There was [a tiny bit of] damage on a corner of [his load] and even if it did kind of clip it, it shouldn't have been able to take a whole bridge down."

Scott was driving the oversized load — a case shed for a drilling rig — behind a pilot truck on the busy highway which connects Seattle and the Canadian border, a route used by 77,000 commuters daily.

Officials are now warning motorists to avoid the area during the coming U.S. Memorial Day holiday weekend.

Scott said her husband did not know he had clipped the bridge.

"It was falling apart behind him. He was looking in the mirror and it dropped out of sight. He just barely made it out," she told CBC News on Friday morning. "It sounded like it was coming down on him."

Scott pulled off the road as quickly as he could, she said.

"He was very scared for what happened," she said. "His main concern was the people."

Two cars plunged 15 metres into the frigid, 4.5-metre-deep Skagit River, putting three in hospital. Police said it was miraculous nobody was killed.

Bridge listed as 'functionally obsolete'

Authorities investigating the cause of the collapse about 100 kilometres south of Vancouver say the semi hit a steel beam on the span, precipitating its collapse into the river.

The National Transportation Safety Board is now leading the investigation into the collapse, said Sgt. Rudeen.

"They're the engineering experts, they're the people that understand bridges," he said. "We don't."

The bridge, built in 1955, has been listed as "functionally obsolete" in a Federal Highway Administration database — a category meaning the design is outdated.

The company that owns the truck Scott was driving is sending people to Washington State, said Ed Scherbinski, vice president of operations for Mullen Trucking, near Calgary..

"We pride ourselves on being one of safest in North America," he said. "We're trying to get to bottom of it, just like everyone else."

Scherbinski said Scott has been with the company for about eight years and was "well experienced on oversized equipment."

Scott had driven the Washington route many times and had all the necessary permits, Scherbinski said.

Scott was interviewed by State Patrol detectives and the patrol's commercial vehicle enforcement bureau troopers until 2 a.m. Friday morning, his wife said.

"He's having a pretty rough go."