(CNN) -- The engineer who was operating the Metro-North train that derailed last year, killing four passengers and injuring dozens more, was suffering from a sleep disorder, according to documents released Monday by the National Transportation Safety Board.
The NTSB's investigation discovered during a post-accident sleep evaluation that William Rockefeller Jr. had "severe obstructive sleep apnea."
In a Metro-North medical history form from 1999, Rockefeller specifically answered "no" to a question asking whether he had difficulty sleeping, and at no point before the accident did any of his health care providers document a discussion with him regarding sleep problems, according to the NTSB's medical factual report of the investigation.
However, after the derailment, a sleep study was ordered for Rockefeller because he "did not exactly recall events leading up to the accident," the report said.
The sleep specialist noted that Rockefeller's recent work schedule change from late night to early morning shifts "might have contributed to the accident," according to an additional statement in the specialist's report.
Sleep apnea is a disruptive sleep disorder in which a person's breathing repeatedly stops and starts. This can result in an individual feeling tired even after a full night's sleep.
As a result, the sleep specialist prescribed Rockefeller continuous positive airway pressure (CPAP) as treatment. Rockefeller reported feeling more energetic after 30 days of treatment.
"My client was fully cooperative with the NTSB investigation, including giving them access to his medical records and doctors," Rockefeller's attorney, Jeffrey Chartier, said Monday. "I believe these reports support that there was no criminality in regards to my client."
The accident happened on December 1 when the train of seven cars derailed while traveling from Poughkeepsie, New York, to Grand Central Terminal in New York City. The train was hurtling along at 82 mph, far above the speed limit of 30 mph for that section of track as it approached a sharp bend in the Bronx.
The cars tumbled off the track, killing four passengers and leaving dozens more hospitalized. The lead car came to rest inches from water at the intersection of the Hudson and Harlem rivers.
At the time of the accident, the train was in "push mode," meaning that the locomotive was in the rear of the train pushing it along the tracks, with Rockefeller in a cabin at the front operating it remotely. His cabin was equipped with a "dead man pedal" that required constant downward pressure with the foot to keep the train moving, MTA spokesman Aaron Donovan told CNN last year.
"Should you lose consciousness or die, and the foot is taken off, a whistle sounds and the train begins to slow immediately to a stop," Donovan said. "It's a pressure you have to keep -- your foot actively engaged."
But it is unclear whether the "dead man pedal" mechanism was activated when Rockefeller allegedly nodded off at the controls. "That is unknown," Donovan said.
"MTA Metro-North Railroad is reviewing the documents released today by the NTSB," Donovan said Monday. "The investigation is still ongoing, and Metro-North will continue to work with the NTSB on addressing their recommendations."
A safety review from the Federal Railroad Administration said Metro-North's focus on punctuality "had a detrimental effect on safety, adversely affecting the inspection and maintenance of track and negatively impacting train operations," according to the report released in March.
CNN's Lawrence Crook III and Haimy Assefa contributed to this report.