The driver of a speeding commuter train that derailed in New York, killing four people, nodded off at the controls before the crash.
Anthony Bottalico, leader of the rail employees union said that engineer William Rockefeller, 46, told investigators he "basically nodded" and said that 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," Bottalico said.
One source involved in the on-going investigation quoted Rockefeller directly as having told investigators, "I was in a daze" in the moments before the crash.
Rockefeller's lawyer did not return calls seeking comment. During a news conference, government investigators said they were still talking to 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.
"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,” Bottalico said.
National Transportation Safety Board (NTSB) 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.
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 and if an engineer does not respond, often by pressing a button, brakes automatically operate.
But the Metro-North train that derailed did not 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 (PTC), which uses electronics to monitor trains' positions and speed and stop derailments and other problems, by the end of 2015.
"For more than 20 years, the NTSB has recommended implementation" of PTC, Weener said. "Since this is a derailment, it's possible that PTC could have prevented it."
Railroad experts have been advocating for PTC systems for years, but they are expensive and complicated and often incompatible for all trains within a single transit system.
The Metropolitan Transportation Authority, which runs Metro-North, said it began work to install Positive TrainControl in 2009 with a goal of implementing it by 2015. The authority said it has budgeted nearly $600m (£370m), with at least another $300m needed, and even then was unlikely to meet the 2015 deadline.