Investigators probing a deadly train crash in Washington State are due to question the driver and a trainee conductor who were in the cab of the locomotive at the time.
They say the possible distraction of the driver is one of the lines of inquiry.
Three people died and dozens were injured when the Amtrak train came off a bridge onto a motorway below.
The train reportedly took a sharp bend at more than twice the speed limit.
Investigators say the emergency brakes were automatically activated during the derailment instead of being manually applied by the driver.
In the video above, NBC Correspondent Tom Costello takes a look at the various issues surrounding the investigation.