The engineer of the Amtrak train that derailed south of Tacoma, Washington, Monday made a comment about “over speed condition” six seconds before the train derailed, according to details released by the National Transportation Safety Board Friday.
While it's not clear, train safety experts say that likely means the engineer realized the train was going too fast just before the derailment.
Related: What We Know: Amtrak Safety Culture In Question After Crash
"I interpret that as, at that point in time he realized he was going too fast with a curve approaching," said Keith Millhouse, an attorney and rail safety expert.
"There's a possibility, although I think it is very remote that there might have been some mechanical reason the train was going too fast."
An initial review of the incident found the train's final recorded speed was 78 mph. The speed limit on the section of the track where the train derailed was 30 mph.
The NTSB's findings are based on the locomotive event data recorder and inward and outward-facing cameras on the train.
Cameras on the train show the crew was not found to be using personal electronic devices before the derailment. The recording also shows the crew bracing for impact on the tilting train before the recording ended.
Amtrak Train Derailment Location
The Amtrak train derailment on Dec. 18 occurred close to where the new Cascades route reconnects to the original route.
Data: US Dept. of Transportation, Open Street Maps, Amtrak / OPB
The engineer did not place the brake handle in emergency-braking mode, the NTSB noted Friday.
"What kind of brake application did he apply? How many pounds of pressure? I would've liked to see that information," said George Gavalla, former safety director for the Federal Railroad Administration, who is now a rail safety consultant.
"There's more that's not being said than that's being said, to me, as someone who's been involved in investigating accidents before."
The NTSB's complete investigation will take 12 to 24 months.
Three people died in the crash and several others were hospitalized.
The railroad company has committed to covering the entire cost of the derailment, from the medical expenses for the injured, to the costs associated with restoring passenger rail service to road repair.
NTSB Chairman Christopher Hart testified before the House of Representatives following the 2015 Amtrak train derailment in Philadelphia. Hart told lawmakers that technology known as positive train control is designed to prevent derailments caused by "over-speeding."
The rail line where the derailment occurred Monday did not have PTC installed, but rather a system called Centralized Traffic Control — or CTC.
Washington state's secretary of transportation told local officials that the Washington Department of Transportation will require positive train control at the location of Monday's fatal derailment before Amtrak Cascades is allowed to resume service there.
Correction: This story has been updated to accurately state the name of CTC, Centralized Traffic Control.