People and vehicles were in the water, KIRO-TV, Seattle, quoted Washington State Patrol officials as saying.
The bridge, between south of Burlington and north of Mount Vernon, collapsed just after 7 p.m., the TV station said.
The Skagit River Bridge is one of 156 state-owned bridges classified as structurally deficient as of Sept. 1, 2011, state Transportation Department records indicate.
Drivers were cautioned to find alternate routes.
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