YONKERS, N.Y. (AP) -- The engineer in the commuter-train derailment that killed four people over the weekend caught himself nodding at the controls just before the wreck, a union official said Tuesday. William Rockefeller "caught himself, but he caught himself too late," said Anthony Bottalico, leader of the rail employees union, relating what he said Rockfeller told him. 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.