A union official says a commuter train engineer caught himself nodding at the controls before the train started to veer off its tracks and derailed in New York City, killing four people.
Union leader Anthony Bottalico said that William Rockefeller "caught himself, but he caught himself too late".
Bottalico says Rockefeller told him he "nodded," akin to a momentary lapse while driving a car.
The National Transportation Safety Board is interviewing Rockefeller. Member Earl Weener says it's too soon to say whether the engineer was fully conscious around the time of Sunday's wreck in the Bronx.
He says it's too soon to say whether the wreck was the result of human error or a mechanical problem.
A lawyer for Rockefeller hasn't returned calls. A former boss calls Rockefeller a stellar employee.
This is a breaking news update. Read AP's earlier story below.
He's worked on the railroad for more than a decade, but engineer William Rockefeller's life may be defined by what he did in the seconds before his speeding commuter train flew off the tracks along a sharp bend.
While investigators have yet to finish talking with him, questions are swirling around Rockefeller because the Metro-North Railroad train went into the curve at 82 mph, or nearly three times the speed limit. Four people were killed and dozens injured.
At a news conference, federal investigators said it was too soon to say whether the wreck was the result of human error - a distracted or sleepy operator, for example - or a mechanical problem.
But they said they have found no evidence so far of any problems with the brakes or signals before the Sunday morning wreck in the Bronx.
National Transportation Safety Board member Earl Weener said Rockefeller was still being interviewed, and Weener would not comment on the engineer's level of alertness as the train hurtled toward the bend.
Alcohol tests on the train's crew members were negative, and investigators were still awaiting the results of drug tests, the NTSB official said.
On the day of the crash, Rockefeller was on the second day of five-day work week, reporting for duty at 5:04am after a typical, nine-hour shift the day before, according to Weener.
"There's every indication that he would have had time to get full restorative sleep," Weener said.
The New York Police Department is conducting its own investigation, with help from the Bronx district attorney's office, in the event the derailment becomes a criminal case.
Rockefeller himself, meanwhile, stayed out of sight. His lawyer did not return calls, but his union and former co-workers spoke up in his defence.
"This is a man who is totally distraught by the loss of life, and he's having a tough time dealing with that," said Anthony Bottalico, his union leader.
He added: "Once the NTSB is done with their investigation and Billy is finished with his interview, it will be quite evident that there was no criminal intent with the operation of his train."
With the NTSB yet to establish the cause of the crash, Gov. Andrew Cuomo said Tuesday the engineer could be faulted for the train's speed alone.
"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.
Rockefeller, 46 and married with no children, has worked for the railroad for 15 years and has been an engineer for 10, according to Weener. Rockefeller lives in a well-kept house on a modest rural road in Germantown, NY, about 40 miles south of Albany.
He started as a custodian at Grand Central Terminal, then monitored the building's fire alarms and other systems, and ultimately became an engineer.
"He was a stellar employee. Unbelievable," said his former supervisor, Michael McLendon, who retired from the railroad about a year ago.
McLendon said he was stunned when he heard about the crash, shortly after opening his mail to find a Christmas card from Rockefeller and his wife.
"I said, 'Well, I can't imagine Billy making a mistake,'" McLendon said. "Not intentionally, by any stretch of the imagination."
Rockefeller's work routine had recently changed. He had begun running that route on November 17, two weeks before the wreck, said Marjorie Anders, a spokeswoman for Metro-North's parent, the Metropolitan Transportation Authority.
Bottalico said Rockefeller had changed work schedules - switching from afternoons to the day shift, which typically begins at 5am - but was familiar with the route and qualified to run it.
In case of an engineer becoming incapacitated, the train's front car was equipped with a "dead man's pedal" that must be depressed or else the train will automatically slow down, Anders said.
Bruno Lizzul, an MTA machinist who met Rockefeller when they both worked at Grand Central around 2000, described the engineer as honest, hard-working and helpful - so much so that he took it upon himself to show up and help Lizzul renovate his home ahead of a baby's arrival.
"He went the extra yard. He just decided to extend himself to me," Lizzul said.
Lizzul said Rockefeller was very serious about his work: "He would not do anything to upset anybody or in any way cause harm."