(USA TODAY) -- The collapse of the Interstate-5 bridge over the Skagit River in Washington State was triggered by a truck with a excessively tall load hitting a steel girder, sending two vehicles into the icy water 50-feet below, Washington State Patrol Chief John Batiste said Friday.
Three people were hospitalized with non-life threatening injuries Thursday evening, but there were no fatalities.
Batiste said the truck, which was carrying drilling rig equipment, struck a bridge support girder, setting off a chain reaction. The vertical clearance from the roadway to the beam is 14.6 feet.
"For reasons unknown at this point in time, the semi struck the overhead of the bridge causing the collapse," Batiste said.
The truck made it off the bridge and the driver remained at the scene and cooperated with investigators.
The 1,111-foot steel-and-concrete bridge, built in 1955, is listed by the National Bridge Inventory as "fracture critical," which means that the entire structure can be brought down if even one major part fails, The Seattle Times reported.
Bridges that have redundant features are designed to remain intact if one a single section is damaged.
Dan Sligh, 47, and his wife Sally were off on a Memorial Day camping trip in their pickup truck when the bridge stretching out ahead suddenly 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 said he dislocated his shoulder but managed to climb out of the vehicle. His wife was knocked unconscious, and he kept her head above water until rescuers arrived more than an hour later.
Emergency teams also rescued one man sitting on top of his car, prompting bystanders to applaud when he reached dry land.
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."
State officials said it could be weeks before the bridge, which carries 70,000 cars a day, can be reopened.
That will be a headache for local and long-dsitance travelrs, as I-5 is the main artery between Seattle, 60 miles south, and Canada.
Batiste urged drivers to avoid the area if possible, especially over the Memorial Day weekend.
"We Washingtonians are going to have to do what we do best, which is to hold together, to show strength of character and a good deal of patience," Gov. Jay Inslee said in a late-night news conference at the scene.
The bridge, which was classified by the NBI as "functionally obsolete," was inspected twice last year and repaired, according to the state Transportation Secretary Lynn Peterson.
The 58-year-old bridge has a sufficiency rating of 57.4 out of 100, according to federal records. That is well below the statewide average rating of 80, according to an Associated Press analysis of federal data, but 759 bridges in the state have a lower sufficiency score.
According to a 2012 Skagit County Public Works Department report, 42 of the county's 108 bridges are 50 years or older. The document says eight of the bridges are more than 70 years old and two are over 80.
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 National Transportation Safety Board announced it is launching an investigation into the collapse.