Troyer said there were "multiple fatalities" but couldn't provide a precise number.
Several motorists in vehicles that were struck by the fallen train cars suffered injuries, but there were no fatalities among people in those vehicles, the sheriff's office said.
At least 77 people were taken to hospitals, authorities said.
An Amtrak spokesperson said there were about 78 passengers and five crew members on board when the accident took place.
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21:02
While the authorities are yet to release a statement on the exact number of casualties in the accident that took place before 8 a.m. local time 64 kilometers south of Seattle, a major emergency response has been underway after the train fell off a bridge that runs above a major interstate.
Media reports indicate that first responders are treating the situation as a mass casualty incident, with several cars still hanging from the bridge.
Photos from the site released by social media users show that a part of the train is apparently hanging off the track.
The ongoing situation involving the Amtrak passenger train that partially fell off a bridge above a highway is "serious," according to Washington Governor Jay Inslee's Twitter statement.
"Thank you to the first repsonders on the scene," Inslee said.
According to Amtrak's statement on Twitter, it is "aware of an incident involving Amtrak train 501," while additional details are due to be released later.
All southbound lanes of Interstate 5 have been closed south of Joint Base Lewis-McChord, with drivers being warned to avoid the area.
According to Reuters citing a news release from the state's transportation department that dates back to October, the accident has occured on the first day that Amtrak trains started using a new track between the cities of Tacoma and Olympia in order to reduce travel time.
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