( 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 .
Engineer of Metro-North train that derailed was suffering from sleep apnea , NTSB report says . William Rockefeller Jr. had no record of sleep disorder before December accident , it says . `` There was no criminality in regards to my client , '' Rockefeller 's attorney says . Four people were killed and dozens were injured in the accident in the Bronx .