(CNN) -- Dan Sligh called out to his wife, who had been sitting on the passenger seat beside him. No answer.
When a section of a bridge over the Skagit River in Washington State collapsed Thursday evening, plummeting into the current dozens of feet below, it took the couple with it. As they pounded onto the river's surface, the impact dislocated Sligh's shoulder.
He popped it back in, groped around the cab of his truck to find his wife and unbuckled her, as the water rose on the passenger side. Then he dragged her out of the truck to safety.
"Rough day," Sligh told CNN affiliate KIRO. "Glad to be here breathing."
What made the section of the 1950s bridge give way is under investigation, but a traffic mishap may have at least weakened the structure, according to police.
"An 18-wheeler commercial motor vehicle with a very large container appears to have struck part of the bridge at some point," said Washington State Trooper Mark Francis. He was not sure of the accident occurred before or while the bridge collapsed.
Sligh thinks there was an accident and that it did trigger the collapse. He was right behind a big rig, when it hit the bridge, he said.
He and his wife had been heading south on Interstate 5, looking forward to a Memorial Day weekend camping trip. They were pulling a trailer behind them.
Not far from the bridge over the Skagit River, they were trailing the semi, he said.
Its wide load looked about four feet too wide for it to clear the side of the truss bridge it was traveling on, he told KIRO. Its cargo jutted out too far to the right, where the bridge's steel beams rose up before crossing overhead.
He saw it coming
That prompted Sligh to comment spontaneously to his wife: "Anytime he wants to go over to the left would be o.k."
Sligh slowed down. "About that same time, another semi-truck came up on the left side," he said, gesturing with his left hand that the second rig was coming up from behind him.
The first rig, bearing the wide load, could no longer veer over, Sligh said.
When the bridge segment dropped out from under his truck, sending it plummeting into the Skagit River, it was a shock to him but not a surprise. Sligh saw it coming.
The 18-wheeler struck the bridge, he said. "There was a big puff of dust, and I hit the brakes."
The bottom dropped out. The water approached from below. "You hold on as tight as you can," Sligh said. A white flash, a flush of water. "The Skagit is definitely cold this time of year," he attested.
The 18-wheeler's driver was questioned but not detained, state police said.
The bridge on Interstate 5 near Mount Vernon was first constructed in 1955, according to CNN affiliate KOMO.
Skagit County, which lies between Seattle and Vancouver, Canada, includes a section of the Pacific coast with many islands.
Of its 223 bridges, the Federal Highway Administration termed 11 "structurally deficient," meaning that they need repairs. Fifty-five were called "functionally obsolete" in the 2009 listing.
It is the less severe category of the two, and the I-5 Skagit River bridge is one of the 55 bearing that designation. The weight of freight crossing over it at one time is limited by law.
"Functionally obsolete" indicates that a bridge's design is from another era; its features are quaint by modern standards. In many cases, lanes are much narrower than those on newer structures.
Neither of the two terms indicates that a bridge is inherently unsafe.
The number of bridges the state's transportation department considers "structurally deficient" has grown nearly 50% in the last six years, according to a 2012 government report. At the time 11% of the "deck area of all bridges" in Washington were in the category.
Some 77,000 vehicles pass through the overhead iron framework of the I-5 bridge daily, a spokesman for the state's transportation department said, and DOT engineers are investigating if an oversized load may have overburdened the structure before it gave way.
Police have asked motorists to avoid the area. With Labor Day Weekend on the horizon, the interstate connecting Seattle and Vancouver, Canada, may see heavy traffic bottlenecking near the collapse site.
When the nearly 60-year-old truss bridge crashed into the Skagit River, it may not have killed anyone, but it may reopen the political debate on the nation's aging infrastructure.
President Barack Obama in late March called for Congress to fund improvements on and construction of new infrastructure. It is a key feature of his budget proposal for 2014, which calls for $21 billion dedicated to that purpose.
"We can't afford Washington politics to stand in the way of America's progress," Obama said in Florida, adding that "ultimately, Congress has to fund these projects."
A federal transportation safety team announced early Friday that it will join the investigation into what made the segment of the bridge collapse at 7:00 p.m. Pacific time (10:00 p.m. ET) Thursday.
CNN affiliate King 5 News in Seattle broadcast images of the collapse, showing a span of the bridge in the water and crowds nearby.
Images included two people sitting atop their vehicles in the water. Sligh was one of them.
When he emerged from his truck, he saw another vehicle across from him, an SUV, half submerged. It's driver told him he was unhurt, Sligh said. Within minutes rescue teams appeared.
The three of them waited patiently at their vehicles to be plucked from the mangled girders of the 1950s bridge.
As the sun went down, the bright green thermal suits of scuba divers on board a speedboat pierced the dusk, as they perused section of bridge that lay collapsed in the waters of a Washington state river late Thursday.
"At this point, we don't have indication anyone else went into the water," said incident management spokesman Marcus Deyerin.
A speedboat with the word "Sheriff" beaming in large, white letters from its side, panned from side to side showing the divers various perspectives of the half-submerged structure, as they prepared to splash into cold current.
They wanted to double-check that no one was trapped under the surface, Deyerin said.
A helicopter hung in the sky above them, and small, red, rescue hovercraft raked the waters nearby. People lined the banks in the amber rays of the dwindling sun to watch them.
No one had been critically injured.
CNN's Jake Carpenter and Dave Alsup contributed to this report
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