(AP) -- A Wal-Mart truck driver who hadn't slept in 28 hours failed to slow down despite posted warning signs and was responsible for a highway crash last year that severely injured comedian Tracy Morgan and killed another comedian, the National Transportation Safety Board said Tuesday.

But the board said the failure of Morgan and other passengers in a limousine-van to wear seat belts and adjust headrests contributed to the severity of injuries when the limo was struck from behind by the truck.

Most of their injuries were caused when the passengers were whipped around or thrown into the sides of the vehicle, the board said at a meeting to determine the cause of the crash and make safety recommendations.

None of the passengers in the back of the 10-seat limo or the driver was wearing a seat belt.

The board said truck driver Kevin Roper of Jonesboro, Georgia, could have prevented the June 7, 2014, crash if he had slowed to 45 mph, the posted speed limit for the construction work zone on the New Jersey Turnpike near Cranbury, where the crash occurred.

The truck traveled 0.9 miles past the first work zone sign and more than 0.4 miles past the 45 mph speed limit sign without slowing from 65 mph. The truck was going that fast until it reached a closing distance of approximately 200 feet before the impact.

At 45 mph, the truck could have stopped before impact, the board concluded.

The collision with the limo started a chain reaction crash that affected 21 people in six vehicles.

"One tragic aspect of roadway deaths is that so often they could have been prevented," said the safety board's chairman, Chris Hart.

Heavy trucks are involved in nearly 1 in 8 fatal crashes, NTSB said. In work zones, 1 in 4 fatal crashes involves a heavy truck.

Roper had driven over 800 miles overnight from Georgia to a Wal-Mart distribution center in Delaware to pick up a load before starting the trip without stopping for sleep.

He had worked for Wal-Mart for 15 weeks and had had nine "critical event reports." Critical event reports, which are generated by a truck's computers and downloaded by Wal-Mart, record things such as hard braking, activation of the vehicle's stability control system or other events that might indicate unsafe driving.

Roper also had been involved in a preventable accident, causing him to lose his safety bonus, investigators said.

Wal-Mart had provided guidance to its drivers on preventing fatigue, but didn't have a comprehensive program to prevent drivers from being assigned over-tiring schedules or to make sure they were rested before reporting to work, the board said. Since then, Wal-Mart has taken greater steps to educate drivers about fatigue and has said it will put in place a program to reduce fatigue, investigators said.

The board has long raised concerns about operator fatigue leading to accidents across all modes of transportation, from airline pilots to train engineers.

Investigators said the limo wound up on its side with its rear doors jammed shut. A sheet of plywood that had been added to the limo to separate the cab from passengers blocked occupants from escaping the vehicle through the front doors after the crash.

It took emergency responders working with the assistance of other motorists 37 minutes to remove the first of the crash victims from the rear of the limo.

Investigators said emergency responders, mostly volunteers, didn't have the training to address some of the logistical and coordination issues arising from the complicated accident. New Jersey doesn't have requirements for the number of training hours that volunteer emergency responders must have, or a certification program, they said.

Comedian James "Jimmy Mack" McNair of Peekskill, New York, a mentor of Morgan's, was killed. Morgan suffered head trauma, and was in a coma for two weeks. Three other passengers in the limo suffered serious injuries.

Morgan, a former "Saturday Night Live" and "30 Rock" star, and the others were returning from a performance in Dover, Delaware.

Roper was charged with death by auto and four counts of assault by auto in state court in New Jersey.

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