The driver whose speeding commuter train derailed along a curve, killing four people, nodded off at the controls before the crash, it has emerged.
William Rockefeller, 46, said he "basically nodded", said Anthony Bottalico, leader of the rail employees union, and by the time he caught himself it was too late.
"He had the equivalent of what we all have when we drive a car. That is, you sometimes have a momentary nod or whatever that might be. How long that lasts, I can't answer that," Mr Bottalico said.
Mr Rockefeller's lawyer did not return calls seeking comment. During a news conference, government investigators said they were still talking to Mr Rockefeller and would not comment on his level of alertness around the time of the Sunday morning crash in the Bronx district of New York.
But two law enforcement sources said the engineer told police at the scene that his mind was wandering before he realised the train was in trouble and by then it was too late to do anything about it. One official said Mr Rockefeller described himself as being "in a daze" before the crash.
Questions about Mr Rockefeller's role mounted rapidly after investigators disclosed on Monday that the Metro-North Railroad train jumped off the tracks after going into a curve at 82mph, nearly three times the 30mph speed limit. In addition to the four people killed, dozens were hurt.
"He caught himself, but he caught himself too late. ... He powered down, he put the train in emergency, but that was six seconds prior to derailment," Mr Bottalico said.
Mr Rockefeller, who was operating the train from the front carriage, was treated for minor injuries. He has worked for the rail company for 15 years and has been an engineer for 10.
National Transportation Safety Board member Earl Weener said it was too soon to say whether the accident was caused by human error, but investigators found no problems with the brakes or signals.
Alcohol tests on the train's crew members were negative and investigators are still awaiting the results of drug tests.
On the day of the crash, Mr Rockefeller was on the second day of a five-day working week, reporting at 5.04am after a typical nine-hour shift the day before, Mr Weener said.
"There's every indication that he would have had time to get full restorative sleep," he said.
Mr Bottalico said Mr Rockefeller "never said anything about not getting enough sleep". But he said the engineer had switched just weeks earlier from the night shift to the day shift, "so he did have a change in his hours and his circadian rhythms with regard to sleep".
New York governor Andrew Cuomo said the engineer could be faulted for the train's speed, if nothing else.
"Certainly, we want to make sure that that operator is disciplined in an appropriate way. There's such a gross deviation from the norm," he said.
Steven Harrod, a University of Dayton professor who studies transport, said trains typically did not have a speed or cruise control, but a power control, which once set, means a train can pick up speed on its own because of the terrain.
"Thus, if the engineer loses attention, the train can gain speed without intervention," Prof Harrod said. "The power control could have been set" as the train left a station, "and then forgotten by the engineer".
In case of an engineer becoming incapacitated, the train's front carriage was equipped with a "dead man's pedal" that must be depressed or else the train will automatically slow down.
Trains also can have alarms, sometimes called "alerters" that sound if the operators' controls have not been moved within a certain timeframe.
If an engineer does not respond, often by pressing a button, brakes automatically operate. But the Metro-North train that derailed did no't have such a system, according to Marjorie Anders, a spokeswoman for Metro-North's parent, the Metropolitan Transportation Authority.
Regardless, "neither of those two methodologies is truly a fail-safe approach", said Grady Cothen, a former Federal Railroad Administration safety official. Congress has ordered commuter and freight rail firms to install technology called positive train control - which uses electronics to monitor trains' positions and speed and stop derailments and other problems - by the end of 2015.