During a late-afternoon news conference, federal investigators said they were still talking to Rockefeller, and they would not comment on his level of alertness around the time of the Sunday morning wreck in the Bronx.
Rockefeller's lawyer did not immediately return calls.
Questions about Rockefeller's role mounted rapidly after investigators disclosed on Monday that the Metro-North Railroad train jumped the tracks after going into a curve at 82 mph, or nearly three times the speed limit. In addition to the four people killed, dozens were injured.
"What he will tell everyone today is that he basically nodded," Botallico said. "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."
Once he caught himself, "he powered down, he put the train in emergency, but that was six seconds prior to derailment," Botallico said.
Separately, two law enforcement officials said Tuesday that the engineer told police who first responded to the scene that his mind was wandering before he realized the train was in trouble, and by then it was too late to do anything about it. One of the officials said Rockefeller described himself as being "in a daze" before the wreck.
The officials, who were briefed on the engineer's comments, weren't authorized to discuss the investigation publicly and spoke on condition of anonymity.
National Transportation Safety Board member Earl Weener said Tuesday that it was too soon to say whether the accident was the result of human error or a mechanical problem.
But he said investigators have found no evidence so far of any problems with the brakes or signals.
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 5:04 a.m. 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.
Botallico said 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."
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. But his union and former co-workers spoke up in his defense.
"This is a man who is totally distraught by the loss of life, and he's having a tough time dealing with that," Bottalico said.
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, N.Y., 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 Nov. 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 5 a.m. — 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."