Story highlights Amtrak train derailed last month near DuPont, Washington, killing three people

Engineer interviewed last week in investigation that could take up to two years

(CNN) The Amtrak engineer on the train that derailed last month in Washington state told investigators he mistook a signal and braked moments before the deadly crash, according to the National Transportation Safety Board.

The Amtrak Cascades Train 501 derailed December 18 near DuPont, Washington, and hurtled off an overpass onto Interstate 5, killing three people and injuring dozens more.

The safety board said Thursday that the 55-year-old engineer, who was interviewed last week after suffering serious injuries in the crash, told the agency the train was traveling at about 80 mph as it passed milepost 15.5 on its inaugural journey from Seattle to Portland, Oregon.

The locomotive engineer was promoted to his position in 2013 after being hired as a conductor nine years earlier. He told the NTSB he planned to start braking about a mile before an upcoming curve with a 30-mph speed restriction at milepost 19.8.

The engineer said he saw mileposts 16 and 17, the NTSB said. But he did not recall seeing milepost 18 or the 30-mph advance speed sign two miles before the curve.

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