The collapse was apparently caused by a semi-trailer truck from Canada hauling an oversize load to Vancouver, Washington, damaging the compression chord in the overhead steel frame (trusswork) on the southbound side of the bridge.
The vertical clearance from the roadway to the upper arched beam in the outer southbound lane is 14 feet 7 inches (4.45 m), and all trucks with over-sized loads are expected to travel in the inside southbound lane where the clearance is around 17 feet (5.2 m).
The trucker was traveling with a 15 feet 9 inches (4.80 m) tall load, and after the collapse a "dented upper corner and a scrape along the upper side [were] visible on the 'oversize load' equipment casing being hauled on the truck."
A pilot car was hired to ensure the load could pass safely.
The pilot car never signalled the truck driver that there would be a problem crossing the Skagit bridge.
The truck fully crossed the bridge as it began collapsing, and the driver remained at the scene and cooperated with investigators.
There were no fatalities, but three people were transported to local hospitals after being rescued from their cars. The cars remained on the bridge deck after it fell into the water.
The trucker, employed by Mullen Trucking, was hauling an over-dimensional load containing a housing for drilling equipment. The company's vice-president, Ed Sherbinski, said permits were issued from Washington State that included clearance for all bridge crossings on the route.
The truck had been led over the bridge by a pilot escort vehicle.
A spokesman for the Washington State Department of Transportation said there are no warning signs leading up to the bridge regarding its clearance height.
Only overcrossings of less than 14 feet (4.3 m) (the normal legal height limit) require advance postings of height restrictions.
The truck driver was interviewed by Washington State Patrol detectives and the patrol's commercial vehicle enforcement bureau until 2 a.m. Friday morning.
The National Transportation Safety Board sent a team for further investigation.
Four company inspectors from Mullen Trucking are traveling to Washington to take part in the investigation.
I-5 Skagit River Bridge collapse - Wikipedia, the free encyclopedia