The engineer who drove the Metro-North train that derailed in the Bronx may have fallen asleep or "zoned out" just before the fatal crash, a law enforcement source said Tuesday.

William Rockefeller could not recall any of his actions until just before the derailment, said the source, who characterized the engineer as being "oblivious" to the fact that the train had built up so much speed at a time when it should have been slowing down.

Investigators believe Rockefeller "zoned out" or fell asleep based on the engineer's own statements, which would explain why he did not hit the brakes until seconds before the crash, the source said.

In light of these revelations, investigators are looking closely at Rockefeller's work logs and schedule to determine if he was getting the proper amount of time off between runs, the source said. They are also seeking to determine when he last slept before Sunday's trip.

Bronx prosecutors, who along with the NYPD and MTA police are conducting a separate probe into the accident, will eventually have to determine whether losing focus or falling asleep at the train's controls amounted to a criminal act that demonstrated recklessness or negligence, the source said.

Investigators expect to finish interviewing the engineer on Wednesday.

NTSB spokesman Keith Holloway said Tuesday that investigators began talking to Rockefeller on Monday but they postponed completing the interview. Holloway wouldn't say why.

The Metro-North train that derailed Sunday morning, killing four people and injuring 63, roared south at 82 mph into a treacherous curve where the speed limit drops to 30, officials said Monday.

A preliminary analysis of the two "event recorders" shows the seven-car train being pushed by a locomotive was traveling at 60 mph two minutes before the crash, but sped up to 82 mph as it went into the curve, National Transportation Safety Board member Earl Weener said in a news conference.

The brakes were applied about five seconds before the 5:54 a.m. train from Poughkeepsie to Grand Central Terminal stopped moving at 7:20 a.m. Cars derailed about 100 feet north of the Spuyten Duyvil station -- ejecting some passengers and nearly plunging into the Harlem River, Weener said.

Using the brakes at that time was "very late in the game" to try to slow down for the curve, said Weener, who is leading the investigation. The train made nine stops before the derailment and there were no problems with the brakes at those times, he said.

Why the train was going faster than the 70 mph limit before the curve is "the question we need to answer," Weener said.

Gov. Andrew M. Cuomo, said in a statement Monday night that the "NTSB's finding that the train was traveling more than double the set speed limit makes clear that, as we suspected, extreme speed was a central cause of this crash."

He said his administration would work closely with investigators and "make sure that any responsible parties are held accountable."

The Bronx district attorney's office, NYPD and MTA Police have opened separate criminal investigations into the derailment, the source said.

This is routinely done in fatal transit accidents in case district attorney officials determine a crime was committed, the source said. The NTSB cannot file criminal charges.

The source said subpoenas have been issued for the train driver's cellphone records, blood samples, drug and alcohol test results and other evidence.

The district attorney's spokesman Steve Reed would not confirm a criminal probe, saying "we're monitoring the investigation by the NTSB and we are part of several agencies involved."

Weener said he could not yet report whether human error or mechanical failure caused the train to derail.

Investigators are still gathering and assessing data, including footage from a surveillance camera that has been sent to a lab in Washington, D.C., to be enhanced. The video, provided by the MTA, was taken from a nearby bridge and shows "flashes followed by a cloud of dust" as the train hit the electrified third rail, Weener said.

Rockefeller, 46, of Germantown, told first responders he had applied the brakes but they did not work, according to the law enforcement source.

Rockefeller said the train had reached an excessive speed going into the turn, the source said. After the brakes did not work, he tried to "dump" them, which is similar to pulling a car's emergency brake, the source said.

Weener said Rockefeller had been interviewed, but declined to reveal what he told investigators, who plan to talk to other crew members in the coming days.

The law enforcement source said Rockefeller's statements would be compared with information from the train's data recorders.

"The data tells the story of how this happened," the source said. "It corroborates or debunks."

Investigators are looking at the 20-year MTA veteran's training and his safety record. They want to know how much sleep he got and the last time he had used his cellphone, which they are examining.

NTSB officials will continue their investigation in New York for several more days, then return to Washington to analyze what they found. A final report could take as much as a year to complete, Weener said.

The four people killed were Donna L. Smith, 54, a paralegal from Newburgh; James G. Lovell, 58, a network television technician from Cold Spring; James M. Ferrari, 59, of Montrose; and Ahn Kisook, 35, a pediatric nurse from Queens.

Investigators found Rockefeller's cellphone, which will undergo routine forensic examining, Weener said. Results of drug and alcohol testing are not complete, he said.

Data from the recorders revealed the throttle went idle about six seconds before the locomotive stopped and the brake pressure fell to zero five seconds before.

Brake air pressure suddenly dropping to zero would indicate that the engineer applied the emergency brakes, but it would have been too late, said David Rangel, founder of the Modoc Railroad Academy, which trains engineers, in Marion, Ill. The momentum of the train would have caused it to continue sliding and skidding at a high speed through the curve, he said.

"If he dumps the brakes, that tells me it's an emergency situation, a crisis situation," Rangel said. "Any modulation of the brakes or tapping of the brakes is not going to be enough."

The Metropolitan Transportation Authority said Monday that engineers are required to test a train's brake system before beginning a run.

Anthony Bottalico, head of the union representing the Metro-North crew, said Monday the four employees who were on the train are cooperating with investigators.

Bottalico called Rockefeller diligent and competent, and said he was traumatized by the derailment. He had been treated and released Sunday from NewYork-Presbyterian Hospital/Columbia University Medical Center.

An assistant conductor in the front of the train suffered an eye injury and broken collarbone, Bottalico said, and the other two crew members were less seriously banged up.

Sen. Kirsten Gillibrand (D-N.Y.) Monday called for the MTA to complete a "comprehensive safety evaluation" of its rail system to prevent similar tragedies.

"Yesterday's accident is the latest in a long list of accidents on MTA's system, and comes on the heels of a freight train derailment near the same turn in July . . . This is simply unacceptable," Gillibrand said in a letter to Federal Railroad Administrator Joseph Szabo.

The NTSB has released the track to Metro-North, but is still inspecting the train cars, which will be moved to a secure facility for further testing.

Sen. Charles Schumer (D-N.Y.), who attended Monday's briefing, called the train's speed "scary."

"It sort of takes your breath away," he said. "For a train to be going 82 miles an hour around the curve is just a frightening thought."