The Interstate 5 bridge over the Skagit River at Mount Vernon collapsed Thursday evening, dumping vehicles and people into the water, the Washington State Patrol said.
The four-lane bridge collapsed about 7 p.m., Trooper Mark Francis said.
Francis said he had no immediate estimate of how many people were in the water or whether there were any injuries or deaths.
He did not know what caused the collapse.
A Skagit Valley Herald reporter at the scene said a sheriff's office rescue boat has arrived and rescue crews were looking for people in the water.
The reporter saw one person sitting atop one vehicle in the water and could see a second vehicle as well.
Crowds of people lined the river to watch the scene unfold, the newspaper reported.