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 on Thursday. Additionally 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. Both the engineer and conductor suffered “serious injuries” in the crash and couldn’t be interviewed until last week, the agency said. A cause has not been determined. The NTSB investigation is expected to take between one and two years.