Dan Sligh could see it coming. An 18-wheeler carrying an oversize load had just struck part of the Interstate 5 bridge over Washington's frigid Skagit River. And the pavement was giving way.

He slammed on the brakes, but the momentum of his truck and trailer carried Sligh and his wife off the bridge and into the water dozens of feet below, he told CNN affiliate KOMO Thursday, hours after the incident some 60 miles north of Seattle.

"You hold on as tight as you can," Sligh said. Then, a "white flash and cold water."

The impact dislocated his shoulder, but the Navy veteran just popped it back in and dragged his unresponsive wife to safety.

Another driver also fell from the bridge, according to state officials. Amazingly, no one died.

"Rough day," Sligh told KOMO. "Glad to be here breathing."

The collapse apparently was the result of a collision between the tractor-trailer's cargo and the bridge, Washington State Patrol Sgt. Kirk Rudeen told CNN on Friday.

As the truck was entering the bridge, it's load -- which Rudeen described as an equipment casing form for a drill rig -- hit a support girder, causing the bridge to collapse.

The bridge had been rated "functionally deficient," according to a federal database, but it was not clear Friday if the bridge's condition played any role in the collapse.

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

"We wouldn't be having drivers drive on this bridge if we thought there were any concerns whatsover," he said Friday.

The National Transportation Safety Board sent a team to investigate the collapse. State police and the state highway department are also investigating.

The collapse happened around 7 p.m. Thursday.

Sligh told KOMO he was traveling south on the interstate behind the tractor trailer when he realized the load appeared to be about four feet too wide to fit through the bridge's superstructure.

"Anytime he wants to go over to the left would be OK," Sligh said he told his wife.

But another tractor-trailer appeared to hem the truck in to the right lane.

"There was a big puff of dust, and I hit the brakes."

Dale Ogden told CNN affiliate KING that he was driving near the tractor-trailer's pilot car when he saw a device on that car designed to indicate whether a truck can clear an obstacle hit the top of the bridge.

He then watched in his rear-view mirror as the truck struck the bridge, he told KING.

"It almost tipped the truck over but it came back down. It tipped it up to about a 30 degree angle to the left and it came back down on its wheels, and almost instantaneously behind that I saw girders falling in my rear-view mirror," he said.

The tractor-trailer did not go into the water. The driver was questioned but not detained, state police said.

The bridge carries about 77,000 cars a day, according to the Washington State Department of Transportation.

It was built 1955, according to KOMO, and was listed by the Federal Highway Administration as "structurally deficient."

That designation means parts of a bridge require monitoring or repair, but does not indicate the bridge is unsafe or likely to collapse, according to the agency. Most deficient bridges remain open to traffic, the agency says.

The number of bridges the state's transportation department considers structurally deficient has grown nearly 50 percent in the past six years, according to a 2012 government report.

Gov. Jay Inslee said the accident starkly demonstrates the need for additional funding for roads and bridges.

"We have some work to do on our bridges whether or not this accident happened, and we have some discussions in Olympia" -- the state capital -- "about making sure that we make investments in bridges to prevent this kind of thing from happening," he told reporters.

In March, President Barack Obama called on Congress to provide $21 billion for infrastructure construction, including improvements to existing roadways.

Michael Pearson and Ben Brumfield wrote and reported from Atlanta; Katharine Barrett reported from Mount Vernon; CNN's Jake Carpenter and Dave Alsup also contributed to this report