Washington ( CNN ) -- A bus crash that killed 15 people and injured 18 others was caused by a `` perfect storm '' of sleep deprivation , speed and lack of oversight , the National Transportation Safety Board said Tuesday . The World Wide Travel motor coach was returning to New York City on March 12 , 2011 , with 32 passengers onboard after visiting the Mohegan Sun Casino in Connecticut . At 5:38 a.m. the bus was on I-95 near the Bronx Westchester County Line , when it crossed a 10-foot-wide paved shoulder , hit a guard rail , skidded 500 feet on its side and slammed into two 8 inch wide signposts that tore off almost the entire top of the bus , according to investigators . The driver , Ophadell Williams , `` was impaired by fatigue at the time of the accident due to sleep deprivation , poor sleep quality and circadian factors , '' according to the report . `` His lack of evasive braking or corrective steering action as the bus drifted off the roadway was consistent with fatigue-induced performance impairment . '' Williams had little rest in the three days before the accident , only taking short naps inside the bus while passengers were in the casino gambling , investigators said . He was also speeding , driving 78 mph in the 50 mph zone in the minute before the accident , according to the NTSB examination of the bus data recorder . Investigators said if the bus had been driving at the speed limit it may not have overturned . At the time of the accident , police said Williams told them the accident occurred after he swerved to avoid a tractor-trailer that might have clipped the bus ; however , the NTSB found no other vehicles were factors in the crash . The report says , `` Contributing to the accident was inadequate safety oversight of the accident driver by World Wide Travel 's management . '' The company has since shut down . Last year , Williams pleaded not guilty to charges of manslaughter and criminally negligent homicide for his role in the accident , according to court records . `` Fatigue and speed are an especially lethal combination , '' NTSB Chairman Deborah Hersman said in a statement . `` Unfortunately , in investigation after investigation , we are seeing the tragic results of fatigue , which can degrade every aspect of human performance . ''
15 died when bus crashed returning from casino in Connecticut . NTSB says bus driver was `` impaired by sleep deprivation , '' had been speeding . Score: 6