Preliminary information obtained from an event data recorder in the rear locomotive showed the train, which was traveling on a new route for the first time, was speeding at 80 miles (128 kilometers) per hour in a 30 mph zone.
But Bella Dinh-Zarr, vice chair of the National Transportation Safety Board (NTSB) told reporters it was "too early to tell" why the train was traveling so fast.
The train, which was carrying 77 passengers and seven crew, derailed in DuPont, about 50 miles south of Seattle, on a curve that passes over busy Interstate 5 at about 7:40 am (1540 GMT).
Pictures from the scene showed one mangled train carriage lying upside down on the interstate highway, while others dangled from the overpass at a precarious angle
Several other carriages also ended up on the highway, shutting down a key section of the busy artery that connects the greater Seattle metropolitan area to Olympia. All but one of the 14 train cars jumped the tracks.
A faster service
Washington State police spokeswoman Brooke Bova confirmed the death toll, and said that of the 100 or so people who were injured, several were in critical condition.
"We don't know if that number will change," she warned.
Officials gave no reason for the derailment of southbound Amtrak train 501, the inaugural run of a new service promising faster connections between Seattle and Portland, Oregon.
Local officials had warned only weeks ago that the track might not be safe enough to handle trains at higher speeds.
Federal investigators would visit the scene on Tuesday to begin an investigation which would last between a week and 10 days, Dinh-Zarr said.
Information obtained from the front locomotive's data recorder, which has been more difficult to access, would potentially lend more insight, she added.
Chris Karnes, a local transit official who was aboard at the time, described the moment when the train veered of the tracks.
"All of a sudden, we felt this rocking and creaking noise, and it felt like we were heading down a hill," he told KIRO-TV.
"The next thing we know, we're being slammed into the front of our seats, windows are breaking, we stop, and there's water gushing out of the train. People were screaming."
The distress call was placed by a conductor on board, who can be heard in an audio recording, saying: "Amtrak 501, emergency emergency emergency, we are on the ground."
"We were coming around the corner to take the bridge over I-5 there... and we went on the ground," he said.
"We got cars everywhere and down onto the highway."
The train hit five vehicles and two trucks, police said.
But there were no deaths among those on the highway below, a spokesman for Pierce County Sheriff's Department said.
Amtrak president and co-chief executive Richard Anderson said he was "deeply saddened" and pledged to do "everything in our power to support our passengers and crew and their families."
The accident marred the launch of faster services on the route after a $181-million upgrade project to improve the rails, the locomotives and the signaling technology.
The trains were expected to reach speeds of 79 mph through the densely-populated area with the improved systems and track.
But local officials had worried about them taking curves in the area at higher speeds.
In early December, local mayor Don Anderson had warned that more needed to be done to ensure safety on the route.
"Come back when there is that accident, and try to justify not putting in those safety enhancements," he said at the time, according to KOMO News.
The local transit official who was on board at the time said the tracks were supposed to have been upgraded to accommodate higher speeds.
"I'm not sure what happened," Karnes said.
Trump pledges investment
US President Donald Trump said the accident underscored the need to invest in infrastructure.
"Seven trillion dollars spent in the Middle East while our roads, bridges, tunnels, railways (and more) crumble! Not for long!" he tweeted.
The new Siemens Charger locomotives were equipped with "positive train control" safety systems, designed to automatically stop the train in dangerous situations and mandated for trains around the country.
But the technology was only expected to be used next year when it is activated on the entire rail corridor, according to the Washington State Department of Transportation.
Dinh-Zarr said investigators were checking to see whether the technology had been installed in this particular track section.
In 2015, an Amtrak train traveling well above the designated speed derailed in Philadelphia, killing eight.
At the time, analysts said positive train control technology could have prevented the accident.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)