He said officials are "scouring the nation" for one of the World War II-era structures known as a Bailey bridge, a type of portable bridge used during by British and American military engineering units during the war.
If one of these bridges is available to span the 160-foot gap — and if it can be held up by the remaining structure — travel on the freeway could be restored in a matter of weeks, Inslee said.
"If not, there are certainly months involved," he said.
The north end of the bridge collapsed just after 7 p.m. Thursday, plunging into the river and taking two vehicles down with it. All three occupants suffered only minor injuries.
A tall truck carrying a load of drill equipment hit several overhead trusses on the bridge just before the collapse. This started a chain reaction in the bridge structure that caused the deck give way, said Travis Phelps, a spokesman for the Washington State Patrol.
"It was definitely the truck," he said.
Inslee said the state will conduct a thorough investigation along with the National Transportation Safety Board. Inslee estimates repairs could cost $15 million.
The stretch of I-5 where the bridge is located carries more than 70,000 vehicles per day. Side streets and state highways will bear the burden of detours and are expected to be very congested in the coming months, starting with this weekend's Memorial Day.
The vertical clearance of the bridge from the roadway to the beam is 14.6 feet. The truck made it across the bridge and the driver remained at the scene and cooperated with investigators.
The tractor-trailer, which was marked as an oversize load, was heading south hauling a housing for drilling equipment to Vancouver, Wash., state trooper Mark Francis said. The top right front corner of the load struck the bridge.
"He looked in the mirrors and it just dropped out of sight," Cynthia Scott, the wife of truck driver William Scott, said from the couple's home near Spruce Grove, Alberta. "I spoke to him seconds after it happened. He was just horrified."
The truck driver works for Mullen Trucking in Alberta. The tractor-trailer was hauling a housing for drilling equipment southbound when the top right front corner of the load struck several of the bridge's trusses, the State Patrol said.
Scott, 41, remained at the scene and cooperated with investigators. He voluntarily gave a blood sample for an alcohol test and was not arrested.
Scott, has been driving truck for 20 years and hauling specialized loads for more than 10.
"He gets safety awards, safety bonuses ... for doing all these checks, for hiring the right pilot cars and pole cars," his wife said.
The U.S. government has promised some aid to help pay for repairs. Washington will receive an initial $1 million, a spokeswoman for Inslee said Friday morning.
U.S. Secretary of Transportation Ray LaHood spoke with Inslee on Friday and said those funds will be released immediately and additional dollars will be available as the reconstruction process moves forward.
Also on Friday, Inslee declared an emergency in Skagit, Snohomish and Whatcom counties as a result of the collapse. LaHood also spoke with members of the Washington congressional delegation.
Rep. Rick Larsen, D-Wash., said the secretary told him an interagency task force has been set up to expedite the permitting process for the bridge repair.
"Members of my staff have been at the scene since last night and are working from the Emergency Operations Center in Mount Vernon," Larsen said in a statement. "My office is coordinating with local, state and federal officials as repairs are planned and executed."
U.S. Sen. Patty Murray, who attended the press conference with Inslee, U.S. Sen. Maria Cantwell, U.S. Rep. Suzan DelBene and other officials, said it's possible the federal government could foot 90 percent of the eventual repair bill.
That level of funding is available for incidents not caused by structural failure, Murray said.
The bridge is classified as a "Warren through truss" steel span, said Debbie Hersman, chairwoman of the National Transportation Safety Board, who spoke at Friday's press conference.
"This is a very common bridge design, especially during the period in which it was built," she said.
The 1,111-foot, steel-truss bridge was built in 1955. It is considered "functionally obsolete," meaning the design is not ideal, but the bridge was considered safe.
State transportation secretary Lynn Peterson wouldn't say if the bridge's age and design will figure into the decision whether to simply repair the gap or to build an entirely new span.
"At this point there's no intent to rebuild the entire bridge," she said.
Federal investigators will spend a week to 10 days in Mount Vernon. The report could take a year to complete.
Meanwhile, the transportation department has set up detours. The nearest bridge, Riverside Drive to the east, 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 dozens of miles to a trip.
"There are actually four bridges across the Skagit River within 10 miles of this one that can be used," Inslee said.
In addition to the headaches and inconvenience, economic impact is a fear as well, officials stressed.
Four days of flooding on I-5 in southwest Washington in December 2007 cost the state $47 million in freight delays alone, according to a transportation department report.
The I-5 crossing at Blaine is the third busiest along the entire U.S.-Canada border, following Detroit-Windsor and Niagara Falls-Buffalo, according to Larsen's office.
Ed Scherbinski, vice president of Mullen Trucking, told The Associated Press that state officials had approved of the company's plan to drive the oversize load along I-5 to Vancouver, Wash.
State officials approved the trucking company to carry a load as high as 15 feet, 9 inches, according to the permit released by the state. The bridge's curved overhead girders are higher in the center of the bridge but sweep lower toward a driver's right side.
The bridge has a maximum clearance of about 17 feet, but there is no signage to indicate how to safely navigate the bridge with a tall load.
The permit specifically describes the route the truck would take, though it includes a qualification that the state "Does Not Guarantee Height Clearance."
Early Friday afternoon, traffic on northbound I-5 was backed up about a mile from the College Way exit in Mount Vernon, where drivers were being detoured. The exit is about a half-mile south of the bridge. A few miles south, traffic was backed up for about three miles behind road crews setting out barrels and signs alerting drivers to the bridge closure.
The riverbank near the collapsed bridge — packed with onlookers on Thursday after the accident — still drew a smattering of curious people on Friday afternoon.
"I take this bridge quite a bit," said Amanda Hertel, 23, of Concrete, who said she frequently visits friends and family in Marysville. "It makes me terrified to be on any bridge at this point."
Three other people could well feel the same way.
Dan Sligh of Oak Harbor and his wife were in their pickup Thursday on I-5 heading southbound to a camping trip when the bridge before them disappeared in a "big puff of dust."
"I hit the brakes and we went off," Sligh told reporters from Skagit Valley Hospital, adding he "saw the water approaching ... you hold on as tight as you can."
The Slighs and Bryce Kenning of Conway, 20, driving a different vehicle, were dumped into the chilly waters of the Skagit River. Kenning was able to kick open the passenger door of his car and climb onto his Subaru's roof.
Sligh said his shoulder was dislocated in the drop into the water, and he found himself "belly deep in water in the truck." He said he popped his shoulder back in and called out to his wife, who he described as being in shock initially as they waited for rescuers to arrive in boats.
Bill Sheets: 425-339-3439; firstname.lastname@example.org.
Herald reporter Gale Fiege, editor Chuck Taylor and the Associated Press contributed to this story.
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