Federal transportation regulators on Tuesday cited a Walmart truck driver’s fatigue as the chief cause of a crash last year that killed the comedian James McNair and critically injured Tracy Morgan, a star of the television series “30 Rock.”

National Transportation Safety Board investigators, presenting their findings at a public hearing in Washington about the June 2014 crash on the New Jersey Turnpike, said unused seatbelts exacerbated the injuries, and criticized the training of emergency medical workers who struggled to remove Mr. Morgan and some of the other six people who were trapped inside an overturned limousine van.

In addition, the limousine had been customized, officials said, leaving the passengers without any available exits until emergency responders cut out part of a plywood panel that had been installed between the passenger compartment and the cab.

“Their single means of exiting had become inoperable in the crash,” the board chairman, Christopher Hart, said.