OLYMPIA, Wash. - The oversize load on a semi-trailer truck that struck a Washington state bridge was likely too tall for the lowest point of clearance on the structure, part of which collapsed after the collision, a federal safety official said on Saturday.

Two vehicles behind the truck plunged into the frigid waters of the Skagit River following the collapse of the span of Interstate 5 on Thursday evening between the towns of Mount Vernon and Burlington, about 55 miles (90 km) north of Seattle.

The three occupants of the vehicles were rescued from the river, but activists and federal lawmakers seized on the partial collapse of the steel truss bridge, which was built in 1955, to call for greater investment in the nation's aging infrastructure.

But Washington state officials have said preliminary indications were that the bridge, which was inspected twice last year, was not structurally deficient and the section fell because of the impact from the truck.

The truck was carrying on its flat bed a large steel, box-like structure built to house drilling equipment, a spokesman for the Washington State Patrol said on Friday.

The truck was permitted by the state to carry a load at a height of 15 feet and 9 inches (4.6 metres), but the lowest point of clearance on the bridge was 14 feet and 6 inches (4.1 metres), National Transportation Safety Board Chairman Deborah Hersman told a news conference on Saturday.

The truck driver has told investigators he repeatedly measured the height of his load at 15 feet, 9 inches, Hersman said. She said the bridge was elliptical in shape, shorter on the sides than in the middle, where the clearance was higher.

Hersman said NTSB investigators planned to measure the load on a flat surface to confirm its height.

PERMIT RULES

While the Washington state Department of Transportation gave Alberta, Canada-based Mullen Trucking, the company that employs the trucker, a permit to carry an oversize load at a height of 15 feet, 9 inches, the state does not provide operators with the vertical limit of each bridge along a route, Hersman said.

The permit, a copy of which the state Department of Transportation posted on its website, says the proposed route for an oversize load "does not guarantee height clearances." A state regulation also posted on the website makes clear the operator is responsible for ensuring the route "is free of overhead obstructions."

As part of that obligation, the company must send a pilot car through to check clearances along the route, Hersman said.

The driver of a pilot car with a pole measuring at least the height of the oversize load did not radio the trucker to warn about the span, Hersman said.

The truck driver has told investigators that his vehicle and the pilot car were both traveling in the right lane, closer to the shorter side of the bridge, as they crossed the span, she said. At the same time, another commercial vehicle was traveling in the left lane, where the clearance is higher, she said. ?

The steel structure the truck was carrying was damaged on the top corner nearest the shortest clearance of the bridge, Hersman said.

David Postman, spokesman for Washington state Governor Jay Inslee, said that with the incident still under investigation, it was too soon to assess blame. "The governor has not pointed the finger," Postman said.

Officials with Mullen Trucking could not be reached by phone on Saturday.