Motorman William Rockefeller, who was also a volunteer fire chief, fell asleep at the helm of fatal Metro-North train that killed four people and injured scores of others. Rockefeller sufferd from sleep apnea, sources said. View Full Caption Rhinebeck volunteer fire department

NEW YORK CITY — The motorman who dozed off while driving a Metro-North train that derailed, killing four people and injuring dozens more last year, suffered from sleep apnea, DNAinfo New York has learned.

Sources said the National Transportation Safety Board, which is investigating the crash that occurred over the Thanksgiving holiday weekend, has found that motorman William Rockefeller suffered from a severe case of the disorder which causes breathing disruptions during sleep.

The ailment deprives a person of a restful night's sleep and can contribute to suddenly feeling tired during their waking hours.

Rockefeller, a 20-year veteran, dozed off at the helm of the ill-fated 5:45 a.m. engine out of Poughkeepsie that was bound for New York on Dec. 3.

The train was travelling at 82 mph when it derailed as it entered a sharp turn at Spuyten Duyvil, sending eight cars cascading toward the Harlem River before skidding to a halt.

Rockefeller cooperated with authorities and claimed he felt fine during the ride to Manhattan and that the last thing he remembered before the accident was a mile marker north of Spuyten Duyvil.

He said he was awakened by the fierce rumbling of the train as it entered a steep curve. He slammed on the brake, but it was too late.

After the crash, in which he was injured, he said he tried to help other injured passengers.

“On the Inside” was the first to disclose that Rockefeller dozed off before the crash and, later, that investigators looking for criminal charges against him found no evidence of wrongdoing.

According to sources Rockefeller was home virtually the entire day before the crash, went to sleep around 8:30 p.m. and was alert when he stepped into the front cab of the 5:45 a.m. Manhattan-bound train.

Investigators determined that he was not on his cell phone — in fact it was turned off — and he was not under the influence of alcohol when the train derailed, according to toxicological tests.

An NTSB spokesman declined to comment.

Sources said, after the crash, Rockefeller underwent a battery of physical tests and physicians discovered he suffered from sleep apnea.

NTSB physicians have confirmed that analysis, sources said.

The sources said the sleep apnea disclosure is expected to be included in a soon-to-be-released NTSB report on investigations into several recent accidents at Metro-North.

Since May 2013, there have been at least four major incidents on the railway, including the Spuyten Duyvil crash. Among them:

► On May 17 2013, two trains collided outside Bridgeport, Conn., after a derailment, injuring more than 50 people.

► On May 28 2013, a train in West Haven, Conn., struck and killed a maintenance worker.

► On July 18 2013, a CSX Transportation freight train derailed crossing into Manhattan from The Bronx.

NTSB officials later determined that the Spuyten Duyvil derailment could have been averted if the train had been equipped with positive control technology to automatically slow or stop trains exceeding speed limits or on collision courses. In addition, the feds have ordered all trains to have two engineers in a cab.

And last month, a blistering Federal Railroad Administration report declared that pressure placed on Metro-North employees to make sure that the trains run on time created a "deficient safety culture."

The FRA investigation, called "Operation Deep Dive," was launched shortly after the fatal December train accident and studied maintenance, inspection and repairs, worker safety measures, communications, train dispatch and traffic control training, compliance with federal work-hours regulations, employee qualifications and other parameters.

As a result of the federal probe, Metro-North's president, Howard Permut, resigned.

New president Joseph Giuillietti acknowledged the system's failure and promised to try to change the railroad's culture.