OLYMPIA, WASHINGTON (REUTERS) - A colliding truck may have triggered the collapse on Thursday of part of a four-lane freeway bridge that sent vehicles and drivers tumbling into a frigid river in Washington state, officials said.
A US National Transportation Safety Board (NTSB) investigation into what led part of the Interstate 5 bridge to fall into the Skagit River 55 miles (90 km) north of Seattle was expected to continue on Friday.
Two of the three people rescued from the river were hospitalised with hypothermia, but no one died, officials said.
The freeway is a principal corridor for vehicles between Seattle and Vancouver, Canada, and Washington state Governor Jay Inslee said he expected major traffic delays in the region.
State Patrol Chief John Batiste said a semi-truck driven southbound struck the bridge just before part of it collapsed.
The bridge has metal overhead beams.
"The size of the load he was carrying appeared to create a problem, causing him to strike the bridge," Mr Batiste said. He said investigators were talking to the driver and inspecting the truck.
State Secretary of Transportation Lynn Peterson told reporters the bridge was built in 1955. It was not among the more than 150 spans the state Department of Transportation listed as "structurally deficient" in 2011.
Local television images showed onlookers gathered at the bank of the Skagit River, calmly watching the rescue attempts under the fallen bridge section.
"The currents of the river are really rough. It's cold,"Barbara Williams, who lives nearby, told Seattle station KOMO-TV.
State lawmakers are debating a proposed US$8.4 billion (S$10.4 billion) transportation funding package that Mr Inslee has championed, along with fellow Democrats. A key point of contention has been whether to pay for a new Interstate 5 bridge over the Columbia River connecting Vancouver, Washington, and Portland, Oregon.
In August 2007, a bridge fell into the Mississippi River in Minnesota, resulting in the deaths of 13 people and raising concerns about faulty infrastructure in the United States.