MOUNT VERNON, Washington (REUTERS) - A bridge collapse that sent cars and drivers plunging into the frigid Skagit River in Washington state, raising concerns about the safety of the nation's aging infrastructure, was caused when a truck crashed into at least one girder, officials said on Friday.

The truck, after the accident, rumbled across the bridge safely before a portion of the structure collapsed, sending two vehicles and a mass of concrete and steel into the river on Thursday evening. Three people had to be rescued, officials said.

While no one was killed, the collapse of the steel truss bridge, which was built in 1955, prompted renewed calls from lawmakers in Washington, DC, and elsewhere for greater investment in the nation's ageing, and in some cases crumbling, infrastructure.

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

"We had a collision between a very heavy vehicle travelling at probably not a small amount of speed crashing into not just one but probably multiple girders, and it failed," Washington Governor Jay Inslee told an afternoon press conference in Mount Vernon.

Officials say the bridge, 90km north of Seattle, was not among the spans listed by the state as "structurally deficient", which in some cases relates to bridges that cannot carry their intended traffic loads.

But the privately run National Bridge Inventory Database listed the bridge as "functionally obsolete", widely defined by public officials as not built to current standards and demands.

"Based on our inspecting, the bridge is not structurally defective," said Ms Lynn Peterson, state secretary of transportation. "We do take hits on almost every one of our bridges. This is just bad luck where and how it was hit."

She said a number of bridges along Interstate 5 in Washington state rated lower than the bridge that collapsed.

A new bridge that suffered a similar blow would have likely reacted the same way, Washington State Department of Transportation spokesman Jan Katzenberger said.

DRIVER COOPERATING

United States (US) National Transportation Safety Board (NTSB) investigators were on the scene of the collapse, which occurred on the four-lane Interstate 5, the principal ighway between Seattle and Vancouver, Canada.

he NTSB said its members would inspect the 339m Warren Truss bridge, which links the towns of Mount Vernon and Burlington, including its substructure, deck and superstructure. Underwater inspections would also be conducted, the agency said.

The truck driver, identified as 41-year-old William Scott, made it across the bridge and pulled his vehicle to the side of the road after the accident. Behind him, a married couple in a pickup fell into the water, as did a man in a Subaru who then sat on the top of his submerged car, authorities said.

Truckers must obtain a permit from the state Department of Transportation to carry an over-sized load. The driver also receives a proposed route from the state, Washington State Patrol spokesman Sergeant Kirk Rudeen said.

An official with Canada-based Mullen Trucking, which employed the truck driver, said state transportation officials had given clearance to take the over-sized loan across the bridge.

"Hopefully we will get some answers," said Mr Ed Scherbinski, vice-president of operations for Mullen Trucking, adding that the company was sending its own investigative team to the scene.

The driver, Mr Scott, has not been arrested and was cooperating with authorities, Mr Rudeen said.

Mr Scherbinski declined to immediately provide the height of the over-sized load the trucker was hauling. The truck had been bound for Vancouver, Washington, he said.

STATE OF EMERGENCY

Mr Inslee declared a state of emergency in Skagit County and two neighbouring counties, citing the disruption of normal traffic for weeks or months, and said it would cost an estimated US$15 million (S$19 million) to fully repair the bridge.

"Now we are on to the business of trying to restore the artery of commerce and economic growth in our state," he said."You cannot overstate the importance of this corridor to Washington state."

Mr Inslee said officials hope to either secure or build a temporary replacement for the 49m span of the bridge that collapsed. It would likely take the form of a prefabricated truss bridge known as a Bailey bridge, he said.

"We are searching the entire country right now" for the right piece of infrastructure, he said.

In August 2007, a bridge fell into the Mississippi River in Minnesota, resulting in the deaths of 13 people and raising concerns about faulty infrastructure in the US.

A 2013 report card from the American Society of Civil Engineers gave US bridges a grade of C+ and ranked Washington state as having the 11th-highest projected cost to perform what the organisation deemed necessary repair or replacement of deficient highway bridges. The cost was pegged at US$2.1 billion.

Washington state lawmakers are debating a proposed US$8.4 billion transportation funding package championed by Mr Inslee and fellow Democrats. A key point of contention has been whether to pay for a new Interstate 5 bridge over the Columbia River connecting Vancouver, Washington, and Portland, Oregon.