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."