The bridge was 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 whatsoever," he said Friday.
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.
"Any time 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 U.S. Department of Transportation released $1 million in emergency funding for the bridge, Inslee said Friday on Twitter. 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. READ MORE: How safe is that bridge you're driving on?