The engineer on an Amtrak train that derailed in Washington state last month, killing three people, told investigators he mistook a wayside signal for another signal before the track speed dropped to 30 mph, the National Transportation Safety Board said in an update Thursday.

In addition to the deaths, more than 70 people were injured when Train 501 of Amtrak's Cascades service from Seattle to Portland derailed and went off a bridge in the city of DuPont on Dec. 18.

An Amtrak train car hangs from a bridge after a derailment on Dec. 18 in DuPont, Washington. Stephen Brashear / Getty Images file

The 55-year-old engineer "recalled that as the train passed milepost 15.5 it was traveling about 79 mph," and said he knew that the curve with the 30 mph restriction was at milepost 19.8, the NTSB said. The engineer said he remembered seeing mileposts 16 and 17, but not milepost 18, less than two miles from the curve, the NTSB reported.

"The engineer said that he did see the wayside signal at milepost 19.8 (at the accident curve) but mistook it for another signal, which was north of the curve," the agency said in an update.

"He said that as soon as he saw the 30 mph sign at the start of the curve, he applied brakes. Seconds later, the train derailed as it entered the curve," the NTSB said. The engineer allegedly told investigators he planned to start braking about one mile before the curve.

Related: 'Positive Train Control' ordered by Congress, but not yet in place

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NTSB member Bella Dinh-Zarr said after the crash it appeared the emergency brake was automatically activated when the accident was occurring rather than being initiated by the engineer, although at the time she noted the investigation was in its early stages.

The accident has highlighted Positive Train Control, a system that among other things is designed to prevent trains from speeding. The train that derailed was not equipped with the system but was scheduled to get it, officials have said.

The train engineer told investigators that he felt rested at the start of his shift and was not distracted by the presence of a conductor who was in the locomotive with him, according to the NTSB. The conductor was there to familiarize himself with the territory.

Related: Washington state Amtrak derailment damage topped $40M, NTSB says

The engineer was hired by Amtrak as a conductor in 2004 and was promoted to locomotive engineer in 2013, and had qualified on the route, the NTSB said. The conductor in the locomotive with him told investigators the engineer appeared alert during a briefing and and while operating the train, the federal agency said.

The conductor, 48, said at the time of the accident he was looking down at copies of track bulletins and "he then heard the engineer say or mumble something. He then looked up and sensed that the train was becoming 'airborne,'" the NTSB said in the summary.

Both the engineer and conductor suffered "serious injuries" in the crash and couldn’t be interviewed until last week, the agency said.

The NTSB said that investigators will work to compare the accounts of the engineer and conductor with video captured by inward- and outward-facing cameras and other information, like that from data recorders.

A cause has not been determined. The NTSB investigation is expected to take between one and two years.

Related: Conductor, passenger injured in deadly Amtrak crash sue

The NTSB has long recommended that all trains feature Positive Train Control, but Congress pushed back a deadline for installation that had been set for the end of 2015, and instead gave train companies until the end of 2018 to put the systems in place nationwide, Dinh-Zarr has said.

Amtrak President and CEO Richard Anderson has said Amtrak "has been a committed PTC leader — prioritizing, completing and activating installation across the parts of the network that we control even as others in the industry have opposed the PTC mandate and worked to delay it."

"It is imperative that the rail industry urgently work together to get PTC activated on the national network as soon as possible — and certainly by the December 2018 federal deadline, if not before," Anderson said in a statement in late December.