Six people killed as train derails onto US motorway
The train derailed just before 1pm GMT (8am local time) on Monday, just south of Tacoma, Washington.
The official, who was briefed on the investigation, also said preliminary signs indicated the Amtrak train may have struck something on the track before derailing.
The official also said that because the tracks were new, it was unlikely the incident was caused by a maintenance issue.
Several vehicles were struck by the falling train cars and “multiple motorists” were injured on the roadway, according to reports.
Amtrak said that there were approximately 78 passengers and five crew members on board the train.
All southbound lanes of Interstate 5 were closed south of Joint Base Lewis-McChord and motorists were being warned to avoid the area.
Chris Karnes was on the Amtrak train that derailed.
Karnes said in a brief phone interview on Monday that he was “not sure what got hit”.
He was unhurt and is now near the medical tents set up in an area between north and sound-bound Interstate 5.
Mr Karnes said: “There are several cars that are hanging over the overpass.”