Cars from an Amtrak train lie spilled onto Interstate 5 near DuPont, Wash., on Dec. 18 (Elaine Thompson/AP)

The engineer of an Amtrak train that derailed south of Seattle last month may have lost track of where he was and thought he had already passed a curve that would have required him to slow the train to 30 mph, according to new details released by federal safety officials Thursday.

The new information comes from interviews with two key witnesses — the engineer and a conductor trainee — who were riding in the locomotive of the Portland, Ore.-bound train when it derailed Dec. 18, killing three people and injuring at least 70 others. Investigators with the National Transportation Safety Board had not previously been able to speak to the men, because of their injuries.

In interviews conducted last week, both detailed what they recalled about the moments before the fatal crash.

The 55-year-old engineer, who has worked for Amtrak since 2004, said he felt rested at the beginning of his shift. He told NTSB investigators he recalled passing several mileposts as he traveled along the route. He said he was aware there was a curve ahead of him with a 30-mph speed limit and had planned to begin slowing the train about a mile before he approached it.

While he saw Mileposts 16 and 17, he told investigators, he did not recall seeing Milepost 18 or the sign advising him about the 30-mph speed restriction, located roughly two miles ahead of the curve. He told officials he recalled seeing a wayside signal at Milepost 19.8, located at the curve where the crash took place, but mistook it for a signal north of the curve. However, he said, when he spotted the 30-mph sign as he was about to enter the curve, he immediately applied the brakes. But seconds later, the train derailed.

First responders work at the scene of the Amtrak crash in Washington state last month. (Daniella Fenelon/AP)

The 48-year-old conductor, training on the route, was hired by Amtrak in 2010. He said he, too, felt rested and alert at the beginning of his shift. He told investigators that there was minimal conversation between him and the engineer and that he had spent his time going over paperwork to help him learn the territory. He said he had been looking at copies of general track bulletins and recalled hearing the engineer say or mumble something. Investigators said he then told them he looked up and sensed the train was becoming "airborne."

The circumstances of the crash in Washington state echo those of a 2015 Amtrak derailment outside Philadelphia that killed eight passengers. In that incident, NTSB investigators concluded that the engineer was distracted by a radio report of rocks being thrown at another train and thought he had already passed a curve that was actually ahead of him. Investigators emphasized they will look into other factors before they determine the cause of this most recent crash. The investigation is expected to take 12 to 24 months.

In a preliminary report released Dec. 22, investigators said the train was traveling roughly 78 mph, nearly 50 mph over the posted speed limit, when it entered the curve and derailed, sending several of its cars spilling onto the highway below. The crash caused an estimated $40.4 million in damage.

NTSB officials said a review of video footage from inside the locomotive showed that crew members were not using any personal electronic devices before the crash. The video also showed that roughly six seconds before the train derailed the engineer made a comment indicating he realized the train was traveling too fast. Investigators said it appeared the engineer applied the brakes shortly before the recording ended; it did not appear that the brake handle was placed in emergency-braking mode.

The train was making its inaugural run on a new route between Seattle and Portland when it derailed near DuPont, Wash., about 60 miles south of Seattle. There were 77 passengers aboard.

Two of the victims — Zack Willhoite and Jim Hamre — were described as close friends and train buffs. The Pierce County medical examiner's office on Wednesday identified the third person as Benjamin Gran, 40, of Auburn, Wash.