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VIDEO: Surveillance Shows Wash. Bridge Collapse

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By Chelsea J. Carter and Katharine Barrett

MOUNT VERNON, Washington (CNN) — The first sign for a truck driver that something was wrong on a bridge on Interstate 5 that collapsed into Washington’s Skagit River was the sound of a “boom,” a federal investigator said Saturday.

At the same time, the driver felt a shudder, NTSB chairwoman Deborah Hersman told reporters.

That’s the moment, according to Hersman, the tractor-trailer hauling an oversized load Thursday struck one of the bridge’s overhead tresses, causing it to collapse and sending vehicles into the river below.

The latest details were released as state officials work to find a temporary solution for the bridge on the heavily traveled interstate near Mount Vernon, about 60 miles north of Seattle.

Heavy holiday weekend traffic along Interstate 5 was reported in both directions at the start of the holiday weekend as cars and trucks inched along a cumbersome detour that took vehicles off the interstate and onto county roads. Roughly 70,000 vehicles travel that portion of the interstate a day, according to the state Department of Transportation.

The NTSB did not release the name of the truck driver. Hersman identified the man as a 41-year-old commercial driver with more than 20 years’ experience — the last eight with Canada-based Mullen Trucking.

He has experience driving oversized loads along the same route, she said.

Oversized load

The problem appears to have begun when the truck with its height of 15 feet, 9 inches, tried to cross the bridge, which has a clearance of 14 feet, 6 inches at its lowest point, according to Hersman.

There was no sign indicating the bridge’s height, she said. Washington state does not require such signs to be posted unless the clearance with a bridge is less than 14 feet, 4 inches.

The 18-wheeler, which was hauling equipment for a drilling rig, was following a pilot car along Interstate 5, Hersman said.

The truck driver picked up his load in Canada, and met the pilot car at the border, she said. The driver of the pilot car was to radio any obstacles the truck driver might encounter.

NTSB investigators plan on interviewing the pilot car driver, she said. The truck made it across before the collapse.

Repair costs

Gov. Jay Inslee estimated that it will take about $15 million to fix the bridge.

Inslee declared a state of emergency in Washington’s northwestern counties of Skagit, Snohomish and Whatcom, noting that the highway is a major conduit and its closure — for a “significant period” — will hurt the area’s economy and its residents.

It’s unclear how long the interstate will be closed for the bridge repairs.

NTSB inspectors were expected to spend seven to 10 days investigating the crash.

As part of the investigation, the NTSB will review the state’s maintenance and inspection records, as well as any damage reports and repairs made to the bridge, Hersman said.

‘Functionally obsolete’

The bridge had been rated “functionally obsolete,” according to a federal database, but state officials said it was safe to drive on.

That category is for bridges that may have narrow lanes or shoulders, or spans that don’t provide enough vertical clearance to let tall trucks pass, according to Washington’s DOT.

The bridge was inspected as recently as November, said Dave Chesson, a spokesman for the Washington Department of Transportation.

Katharine Barrett reported from Washington and Chelsea J. Carter reported and wrote from Atlanta. CNN’s Jackie Castillo contributed to this report.