The National Transportation Safety Board released a stinging report today on a deadly string of Metro-North Railroad accidents.

The NTSB announced its probable causes for five accidents between March 2013 and May of this year.

Officials said that one of the accidents, the deadly train derailment in the Bronx on Dec. 1, 2013, happened because the engineer, William Rockefeller, fell asleep due to undiagnosed sleep apnea and a drastic change in his work schedule.

The NTSB also said the derailment and collision in Bridgeport in May 2013 that injured dozens was caused by broken joint bars used to join different rails.

The NTSB said the lack of a comprehensive track maintenance program contributed to the crashes. Investigators say the Metro-North was relying on visual maintenance rather than physically inspecting the tracks.

As for the CSX derailment in the Bronx in July 2013, the NTSB said Metro-North was seven years behind schedule on track maintenance.

The Metro-North said it did not have a policy to screen employees for sleep disorders, but that it will starting in December. The railroad will start to physically check tracks and will also start to outfit trains with Positive Train Control - a device that automatically stops a train when it goes over the speed limit.

The Metro-North said it is dedicated to safety and learning from the past.