Investigators looking into the fatal derailment of an Amtrak train near Tacoma, Washington, say they will be focus on the speed of the train as the possible cause.
The Seattle-to-Portland train, in its inaugural run on a new route, was going 80 mph around a curve that had a speed limit of 30 mph.
"That's one of the things we're going to be investigating," said Bella Dinh-Zarr, vice chairman of the National Transportation Safety Board.
Thirteen of the train's 14 cars and engines left the rails Monday, including one that crashed down on cars traveling on Interstate 5.
At least three people were killed and nearly 100 suffered injuries.
The Seattle Times noted that Russell Quimby, a consultant who previously investigated for the NTSB, pointed out that the track appeared relatively undisturbed, but the train drove in a straight line, missing the turn.
The Washington Department of Transportation confirmed that the speed limit for trains drops from 79 mph just before the curve and bridge where the accident happened. Signs are posted two miles in advance, and again at the site, officials said.
Train 501 was on its first run along the route since a $181 million project that was intended to speed up service by moving the trains to rails that have fewer curves and less traffic.
Mary Schiavo, a transportation analyst for CNN, speculated that the combination of the speed and the curve will be an important factor.
"This train was about to enter or was entering a curve and while they had to modify the tracks and test the tracks – and all of this work was done at the beginning of December – local officials in Washington were highly critical of sending a train at this speed through this area … they specifically warned that it needed to slow down at the curves.
"I always like to say, whether it's a train crash or a plane crash, the laws of physics are the only laws you can't break. And while they tested it … testing as opposed to running a full-sized, fully loaded train over the track changes the physics. It changes the dynamics of the forces that you have in that curve.
"It's like racing a motorcycle. As you approached that curve, the centrifugal forces on the train change dramatically, and I bet the NTSB is gonna pay a lot of attention to the topography and whether the train was entering a curve."
The engineer notified dispatchers of the emergency when it happened.
"Amtrak 501 emergency, emergency, emergency, we are on the ground," he said.
"Is everybody OK?" the dispatcher asked.
"I'm still figuring that out," said the engineer.
Officials confirmed that a new feature, called positive train control, which can automatically slow trains to avoid danger, was installed along the route but not activated yet.
The derailment left train cars dangling over the interstate's southbound lanes, which were closed.
Pierce County sheriff's Det. Ed Troyer said the train struck several vehicles on Interstate 5, but nobody in the vehicles died.
Maria Hetland was on the highway when she came upon the blockage.
"It was awful," she told the Seattle Times.
She said there were people walking on the roadway and more sitting on the side of the road wrapped in blankets.
"I don't know how that train fell off that bridge. There's a whole railing, a wall," she said. "I don't know how it crashed through the wall," she told the paper.
Local television station KING TV talked with Greg Mukia, who was on the highway.
"I looked up and, obviously, super scary, train had come off the overpass there and parts of it were kind of hanging. Lots of smoke. People running to try to help."
CBS News reported Amtrak announced service from Seattle to Portland had been suspended.
A list published by a Fox affiliate this week showed speed has played a key role in several recent derailments.
On April 3, 2016, an Amtrak going more than 100 mph in Chester, Pennsylvania, derailed and struck and killed two maintenance workers.
On May 12, 2015, another Amtrak, running near Philadelphia, derailed and left eight dead and more than 200 hurt when it hit a sharp curve going twice the 50 mph speed limit.