SEATTLE — A large section of a bridge on Interstate 5 north of Seattle collapsed Thursday evening, sending vehicles and people plunging into the swirling, frigid waters of the Skagit River.
Three people were hospitalized in stable condition, officials said. No one was killed.
The bridge failed without warning between the towns of Burlington and Mount Vernon on the major route linking Seattle with the Canadian border, the Washington State Patrol said.
Several witnesses told local TV that a truck carrying an oversize load had crossed the bridge and struck it just before the collapse. Other witnesses saw girders falling. The truck's load was about 12 feet wide by 14 feet high, Dale Ogden, a trucker who was nearby, told Northwest Cable News.
PHOTOS: Bridge collapse in Washington
After vehicles fell into the water, at least two people could be seen in photographs atop their vehicles, surrounded by the twisted steel beams of the bridge.
Initially, State Patrol officials said they could not say how many vehicles fell into the river until they did a complete inspection. A sheriff's rescue boat was on the scene and crews were looking for people in the water as helicopters hovered overhead.
Hours later, the Skagit County sheriff said that two vehicles had fallen in, and that three people had been rescued.
Jacob Matson, who was in a nearby gym, said a loud crashing sound could be heard as the structure collapsed.
"It had just fallen into the river," he said. "It just kind of sounded like a boom, a thud."
Emerson Shotwell and Christian Sweet were headed north on the freeway when traffic came to a sudden halt about 7 p.m.
"Everyone had to slam on their brakes," said Shotwell, 24, of Puyallup, Wash. "From 60 to zero."
Sweet, 22, of Bonney Lake, Wash., said they were probably just seconds from driving onto the bridge.
PHOTOS: Bridge collapse in Washington
He said that had they been running just a little earlier, "we'd be in the water right now."
About an hour later, the two sat in their car a few hundred feet away. Traffic was still stalled, and the bridge was blocked off by authorities, Shotwell said. He and Sweet could see ambulances and police cars ahead, and they could hear sirens and see firetrucks minutes after the collapse.
Later, they joined the crowd assembled on the bank, which erupted into applause as rescuers pulled a young boy from a pickup that was in the water. Shotwell said he saw that the boy was being treated on the riverbank, but he could not see how severely the child was injured.
Bart Treece, a spokesman for the Washington State Department of Transportation, said the bridge was built in the 1950s but was regularly inspected, most recently in November.
"We're still trying to piece together what happened," he said. "Our concern right now is getting people north and south, as I-5 is a pretty heavily traveled route."
According to the National Bridge Inventory, a database compiled by the Federal Highway Administration, the bridge was built in 1955 and was deemed "functionally obsolete" as recently as 2010. That does not mean it is unsafe, however. It could mean that its design is outdated — that its lanes or shoulders are too narrow, for example.
The American Society of Civil Engineers' 2013 infrastructure report card gave Washington state a C overall and a C- for its bridges. The state's own report said about 95% of its bridges were rated as good or fair, with 5% considered poor, as of 2011.
Repairing and updating deficient bridges have been part of the national conversation at least since 2007, when the I-35W bridge collapsed in downtown Minneapolis during evening rush hour. Thirteen people were killed and 145 hurt.
PHOTOS: 2007 Minnesota bridge collapse
President Obama included $28 billion for bridge and highway repair in his $787-billion stimulus measure when he took office in 2009. More recently, he called on Congress to pass a $21-billion measure to bolster the nation's infrastructure.
In March, he told a Miami audience: "We've still got too many roads that are in disrepair, too many bridges that aren't safe."
Murphy reported from Seattle, Rojas from Los Angeles. Stephen Ceasar and Cathleen Decker in Los Angeles contributed to this report.