Washington (CNN) -- The NTSB continues to rule out mechanical causes in a deadly Bronx train crash while adding to the possibility that human error was involved.
Federal safety investigators have all but eliminated mechanical problems as the cause of the December 1 derailment but added one more clue suggesting the engineer might not have been as attentive as he should have been.
At 7:11 a.m. that Sunday -- about 11 minutes before the derailment -- the engineer did not dim the train's headlights as required when it passed another train, the National Transportation Safety Board said Monday.
Engineer William Rockefeller Jr. acknowledged to investigators having a lapse in attention as the train approached a sharp curve, according Rockefeller's lawyer, Jeffrey Chartier, and his union representative. The train's motion might have caused him to nod off -- a case of what the lawyer termed "highway hypnosis."
Four people died and dozens were injured when the train, traveling at about 82 miles per hour, derailed on the curve, which has a 30 mph limit.
The NTSB said Monday it believes that positive train control, a high-tech system that targets human error, could have prevented the crash. The safety board has pushed the system for about 20 years. Congress is requiring most major railroads, including Metro-North, to install the costly systems by the end of 2015.
The NTSB said it has completed its inspection of the train and found no anomalies.
The "dead-man" switch -- a foot pedal that must be depressed to keep the train moving -- was also working, the board said. Nor did it find problems with speed sensors, the brake control unit or the train's propulsion controller.
The board reiterated that it found no problems with the tracks or the signal system.
Finally, the board said it conducted a "site/distance" test and found no problems with visibility.
"As a result, at this time, the NTSB believes that if positive train control technology was installed on this line and train, it would have required the engineer to slow the train to an appropriate speed or stop the train in the event the engineer did not do so, likely preventing the derailment," the board said in a statement.
Investigators are still awaiting results of crew drug tests and cellphone records, the board said, and it is continuing interviews with passengers, Metro-North employees who were riding on the train and first responders.