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The National Transportation Safety Board cited driver fatigue as one of the factors that caused the New Jersey highway crash that injured Tracy Morgan and killed one of his friends.

The board said Tuesday that Walmart truck driver Kevin Roper had not slept in 28 hours when the crash occurred, according to the Associated Press, and could have prevented the accident if he had slowed to 45 mph, the posted speed limit for the construction work zone on that part of the New Jersey Turnpike.

Investigators also noted that Morgan and the other passengers in the 10-seat limousine-van he was riding in were not wearing seatbelts, which they said contributed to the severity of injuries sustained when the truck struck the limo from behind on June 7, 2014.

Morgan was severely injured in the crash, which claimed the life of fellow comedian James “Jimmy Mack” McNair. Three other passengers in the limousine also suffered severe injuries, the AP reports.

The former Saturday Night Live and 30 Rock star settled a lawsuit against Walmart in May stemming from the crash. A month later, he gave his first televised interview since the accident, mourning McNair’s death and admitting he was not yet ready to return to performing.

“Bones heal, but the loss of my friend will never heal,” he told Matt Lauer on the Today show, adding, “I love comedy. I’ll never stop loving her. I love comedy and I can’t wait to get back to her. But right now my goal is just to heal and get better. I’m not 100 percent yet. I’m not. And when I’m there, you’ll know it. I’ll get back to making you laugh, I promise you.”