- Video report by ITV News Correspondent Duncan Golestani
The Amtrack train that derailed near Seattle killing at least three people was travelling at 80mph in a 30mph zone, according to a spokeswoman for the US National Transportation Safety Board.
An event data recorder recovered from the train showed it was travelling at 80mph, NTSB board member Bella Dinh-Zarr said, but she told journalists it was "too early to tell" why the train was travelling at that speed.
"It's a very complicated accident," she said.
NTSB officials are investigating the incident, which saw a train carrying 80 passengers and five on-duty crew members come off the rails onto a busy interstate road near Seattle in Washington state on the west coast.
Train 501 was making its maiden trip on a new high-speed rail link between Portland and Seattle at around 8am local time (1pm GMT) on Monday.
Three people have been confirmed dead, but there are fears that number could still rise.
Ed Troyer, a spokesman for the Pierce County Sheriff, said there were some train carriages were too unstable to search.
Five vehicles were crushed by the falling train carriages but no one travelling on the road had died, Troyer said.
More than 70 people have been taken to hospital, with 10 people seriously injured. Medical officials said that at least two of the injured were in a critical condition.
Mr Troyer dismissed theories as to why the crash had occurred as "just speculation", but following the NTSB's revelation it appears the train was travelling well above the speed limit on that part of the track.
A track chart prepared by the Washington State Department of Transportation shows the maximum speed drops to just 30mph for passenger trains as the tracks curve to cross Interstate 5, which is where the derailment occurred.
An Amtrak official said the train was not using a technology called positive train control (PTC), which can prevent derailments caused by excessive speed.
Ms Dinh-Zarr said investigators were still looking into this. Responding to questions from journalists she said PTC could prevent "types of accident such as this ... [but] we should remember that PTC can't prevent every accident".
Investigators are also examining whether there was an obstruction on the track.
Witness Daniel Konzelman, 24, was driving parallel to the train on his way to work in Olympia. He was about 30 seconds ahead of the train on the freeway when he saw it derail.
He pulled off the road and rushed to climb inside the mangled train to help the injured.
Mr Konzelman said some people appeared to be dead, while others were alive but trapped under the train. He helped those who were able to move escape and talked to others in an effort to comfort them.
"I prepared for the worst and hoped for the best. I saw a little bit of both," he said.
"I just wanted to help people because I would want people to help me."
President Donald Trump said his "thoughts and prayers" were with everyone involved in the accident and the White House was monitoring the situation.
In first tweet on the accident, he said it "shows more than ever why our soon to be submitted infrastructure plan must be approved quickly" as he said railways and roads had been left to "crumble".
"Not for long!" he added.
Jenny Durkan, the mayor of Seattle, said her thoughts were with all those caught up in the "catastrophic" accident.
Amtrak President and co-CEO Richard Anderson said everyone at Amtrak was "deeply saddened" and said they would work to support passengers and crew caught up in the accident.
All southbound lanes of Interstate 5 were closed south of Joint Base Lewis-McChord, and motorists were warned to avoid the area.
The scene has been preserved for investigators and Amtrak trains running south from Seattle were suspended on Monday as emergency services dealt with the accident.
The accident happened on the first day of a the new Amtrak Cascades' high-speed route between Seattle and Portland.
It saw trains moved onto a new inland route which runs in a straighter line than the previous track, meaning that trains could travel faster.
The new route, called the Point Defiance Bypass Project, was touted as an improvement over lines that hugged the scenic Puget Sound but included tight curves and single-track tunnels.