WASHINGTON, D.C. — A severely fatigued motorcoach driver who lost control of the vehicle was among the probable causes of a fatal motorcoach crash last May, the National Transportation Safety Board (NTSB) reported.

The agency also cited the failure of Sky Express Inc., the company operating the motorcoach involved in the crash, to manage safe driving practices, and a lack of adequate regulatory oversight as contributing factors to the crash.

On the morning of May 31, 2011, a 2000 Setra motorcoach was traveling on an interstate in Doswell, Va., en route from Greensboro, N.C., to New York when it crossed the rumble strips and traveled onto the right shoulder, striking a cable barrier and overturning onto its roof. Four of the 58 passengers were killed and 49 others were injured.

"This crash should never have happened," NTSB Chairman Deborah Hersman said. "It was entirely preventable. Those travelers were failed at three levels: by the driver, the operator and the regulator."

An examination of the driver's work schedule, sleep times and cell phone use revealed that his opportunity for sleep in the 72 hours prior to the crash was limited, resulting in what the NTSB described as "acute sleep loss, poor sleep quality and circadian disruption."

The NTSB also found that Sky Express Inc. management failed to follow adequate safety practices and exercise safety oversight of the driver. The agency cited the Federal Motor Carrier Safety Administration (FMCSA) as contributing to the accident because of lax safety oversight of Sky Express and repeated failure to enforce federal safety regulations against the company.

This failure resulted in Sky Express continuing its operations despite the existence of serious safety issues, NTSB said. Sky Express was only operating at the time of the crash because of an additional temporary extension granted to the company by FMCSA. The agency removed the carrier's operating authority after the accident.

"You have to ask why an overburdened regulator, like FMCSA, with resources to conduct compliance reviews on only 2-3% of operators each year, would visit the same operator year after year?" Hersman added. "And even more to the point, given all the reviews that identified a myriad of safety deficiencies, why was Sky Express still operating?"

As a result of its 13-month-long investigation, the NTSB made three new safety recommendations to FMCSA and also reiterated and reclassified previous recommendations to the National Highway Traffic Safety Administration and FMCSA.

A synopsis of the NTSB report, including the probable cause, findings and a complete list of the safety recommendations, is available here.

The full report will be available on the NTSB website in several weeks, officials said.

Other recent news related to the NTSB:

● NTSB calls for speed limiters on trucks, buses

● Distracted driving: high on the national policy agenda

● School buses excluded from stability control proposal