The Alberta trucker who struck a steel beam precipitating a bridge collapse in Washington State was hauling an oversized load that prevented him from crossing the Skagit River bridge safely, according to a U.S. transportation official.

Deborah Hersman, chair of the National Transportation Safety Board (NTSB), said 41-year-old William Scott was granted a permit for his oversize load — a case shed for a drilling rig — which listed the cargo as 4.85 metres in height.

Scott told officials he measured his load multiple times along his route and was following a pilot truck whose driver was responsible for warning him, through a two-way radio, of any problems the oversized trailer might encounter.

"The purpose of having a pilot car to travel ahead of the tractor trailer with this vertical clearance issue is to ensure the tractor trailer can pass safely," Hersman said on Saturday.

Deborah Hersman, chair of the National Transportation Safety Board said her team will spend about a week inspecting the I-5 bridge. (CBC)

But Scott told officials the pilot driver never signaled him that crossing the bridge over the Skagit River would be a problem, even though the bridge’s lowest point only had a clearance of 4.45 metres.

There are no signs describing the bridge’s vertical clearance, as signage is not required in Washington State unless the bridge has a clearance less than 4.39 metres.

Scott told officials he had worked with the pilot truck company before, but it was the first time he worked with this particular driver.

Concrete slid off girders "like icing sliding off of a cake"

Three people were injured on Thursday when the bridge collapsed, sending two vehicles into the Skagit River near Mount Vernon on Interstate 5, a busy highway that connects Seattle to the Canadian border.

Hersman said Scott "described hearing a boom and feeling contact in the vehicle" as he drove over the bridge.

"From the bending and the buckling, the bridge is telling our investigators the story of where the failure originated and how it propagated through the span that dropped into the water," said Hersman, who told reporters multiple teams have been on site since Thursday to investigate the bridge structure.

"You can see that the concrete section of that dropped span has slid off of its girders like icing sliding off of a cake. It’s a floating bridge deck and it has completely slipped in certain sections right off of the deck."

Regulators have dubbed the steel bridge, which was built in 1955, as a "fracture critical" structure, which can crumple when a single, vital component is compromised.

Other large vehicles have struck the Skagit River bridge before the collapse Thursday, Hersman noted.

Bridge could be closed for months

Hersman says teams will spend about a week inspecting the I-5 bridge, interviewing the driver of the pilot truck, and examining maintenance documents and accident reports.

Investigators will also be using a high tech 3-D video camera to review the scene and attempt to pinpoint where the bridge failure began. Officials say they are still working to find out whether the collapse was a fluke or a sign of bigger problems.

"The results can be very catastrophic," Hersman said. "We're very fortunate in this situation."

State and federal officials will work together on the investigation, said Hersman. They'll also be on the lookout for safety issues that could affect other bridges.

In the meantime, motorists should not expect to drive on I-5 between Mount Vernon and Burlington for many weeks and possibly months, said Washington Transportation Department spokesman Bart Treece.

About 71,000 vehicles use that stretch of highway every day.

However, officials are looking for a temporary, pre-fabricated bridge to replace the 48-metre section that failed, Washington Gov. Jay Inslee said Friday. That option could be in place in weeks.

Otherwise, it could be months before a replacement can be built, said Inslee, adding it will cost $15 million to repair the bridge.