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At a briefing in Washington, D.C. on Tuesday, the National Transportation Safety Board concluded bus driver Ophadell Williams was over-tired when he crashed into a guard rail on the New England Thruway on March 12, 2011.

Fifteen people were killed.

The bus was returning to Chinatown after an overnight trip to a Connecticut casino.

Officials say Williams was driving 78 miles per hour, nearly 30 miles over the speed limit, right before the crash.

Williams claimed he had enough sleep in the hours before driving the bus, but that was contradicted by cell phone and rental car records, which showed he had been awake.

"All of the signatures that we saw in this accident, the slow drive off the road, the lack of responsiveness or breaking, tell us that this driver was very fatigued - and likely asleep," said NTSB Spokeswoman Deborah Hersman.

Williams pleaded not guilty to charges of manslaughter and criminally-negligent homicide.

The board also faulted the bus company, World Wide Tours of Greater New York, for not conducting proper oversight.

The company was shut down after the crash, but officials say they continued to operate under a new name.

Federal officials later made more than a dozen recommendations for bus safety, including better devices to limit a coach's ability to travel over a certain speed.