Story highlights Truck driver was awake for 28 consecutive hours, NTSB says

He didn't exceed work hours, but poor rest beforehand was factor in crash, board says

Board: Passengers didn't use seat belts, which contributed to injuries

(CNN) A truck driver's fatigue -- exacerbated by his decision to drive 12 hours to work before he even started his 14-hour shift -- likely caused the June 2014 crash that killed comedian James McNair and seriously injured comedian Tracy Morgan in New Jersey, federal officials said Tuesday.

But McNair, Morgan and others in the passenger compartment of a limo van weren't wearing seat belts or using properly adjusted head restraints, a factor that contributed to the severity of their injuries, the National Transportation Safety Board said.

A Walmart truck driven by Kevin Roper rear-ended the limo van, which was carrying seven people, shortly before 1 a.m. on the New Jersey Turnpike on June 7, 2014.

The crash killed McNair -- known as "Jimmy Mack" -- and seriously injured four other passengers, including former "Saturday Night Live" and "30 Rock" performer Morgan, who was left with a brain injury.

The NTSB said Roper was driving 65 mph in a work zone with a posted 45 mph limit, and failed to brake in time for slow-moving traffic ahead of him, including the limo van.

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