Witnesses say Hart stepped into the elevator on her way to work at 10 a.m. on Wednesday. The door didn’t close, and the elevator shot straight up, trapping her lower body between the cab and the wall. The elevator has a number of redundant safety devices. The doors rely on motion sensors that indicate a passenger is entering and prevent the door from closing. And an electromechanical interlock is designed to keep he cab from moving until the doors are completely closed. In this case, because witnesses say the door did not close before the cab moved, sources say it indicates the motion sensor and the interlocking device failed. Among other areas of inquiry, investigators are trying to determine whether the circuit board replacement contributed to a sudden power surge that overrode those safety devices just as Hart entered the cab, according to the two sources with direct knowledge. The industry source described such a surge as “generator suicide,” referring to the elevator power source. The source said such surges typically cause little harm because they occur when doors are closed and result in passengers experiencing a sudden upward jerking motion. If such a surge occurred just as Hart entered the elevator, the source said, the timing “was a billion to one. It’s incredible.” On Friday, the three agencies looking at the accident — the city Buildings Department, the city Department of Investigation, and the U.S. Occupational Safety and Health Administration — declined comment.