The Interstate 5 Skagit River Bridge closed for several hours Wednesday morning after a truck carrying liquid nitrogen crashed into a barrier at the southbound bridge approach.

It was nearly the same spot as a bridge collapse May 23, 2013, when an overheight load of drilling equipment struck an overhead brace, setting off a chain reaction to break the steel truss. Three motorists were thrown into the water with the falling deck that day seven years ago but miraculously suffered only minor injuries.

Bridge engineers Wednesday morning, jittery about reports of “frost” from a nitrogen spill, diverted traffic as a precaution, until a team could inspect the steel trusses and deck girders from a lift bucket.

The crash happened just after 4 a.m., when the southbound truck veered into a metal guardrail, then continued onto the bridge where it scraped the concrete side barrier. The force knocked the front axle loose, said Trooper Heather Axtman, a State Patrol spokesperson.

By the time state inspectors from Tumwater arrived, around 9:20 a.m., they couldn’t even pinpoint the spill location through the mist, because the nitrogen was apparently washed out by rainfall, said Evan Grimm, bridge-preservation engineer for the Washington State Department of Transportation (WSDOT). But inspectors confirmed the bridge was safe to reopen, he said.

“We don’t have a lot of basis for understanding the behavior of liquid nitrogen on the structures,” Grimm said. Nitrogen travels in tankers at about minus-320 degrees Fahrenheit and can escape rapidly as a gas.

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Traffic resumed at 11:15 a.m., after hours of delays where drivers detoured through freeway exits to Highway 20 and George Hopper Road.

Wednesday’s closure disrupted the main public and trucking route between Seattle and the Canadian border. The four-lane bridge carries an average 71,000 daily vehicles.

Workers didn’t see dents or obvious structural damage to the steel, WSDOT spokesperson Bart Treece said.

That’s important because the bridge, built in 1956, is “fracture-critical” — meaning a failure in one area can unleash forces that ruin a whole span, as happened in 2013. The northernmost span of the 1,100-foot-long crossing is a concrete-girder section built after the 2013 incident, while the other three are supported by overhead steel trusses.

WSDOT solved its clearance problem after the collapse, by sawing and replacing the overhead braces, raising the room overhead from 15 feet, 6 inches to a full 18 feet at a cost of $4.5 million.

However, this week’s crash raises questions about whether a slightly different truck path would have destroyed the bridge by a direct horizontal strike. Grimm replied the concrete barriers are fastened into the freeway deck and designed to deflect all vehicles, rather than slide sideways on impact. WSDOT hasn’t encountered any incidents where a heavy truck pushed freeway barriers into the fracture-critical bridge members, he said.

Despite its role as an example of U.S. infrastructure neglect, state lawmakers didn’t provide money to replace the crossing in a 2015-16 gas-tax increase, or a potential 2021 increase. Instead, money is flowing to add lanes across the growing state, to pay debt for a new Highway 520 bridge and other megaprojects, to reopen blocked salmon streams, and to patch crumbling pavement statewide.