WASHINGTON -- The driver of a Wal-Mart tractor-trailer that slammed into a limo carrying comedian Tracy Morgan last June had been awake more than 28 hours, the National Transportation Safety Board said Tuesday.The crash happened around 1 a.m. on June 7, 2014, when the 35-year-old driver slammed into Morgan's limo from behind, killing comedian James McNair and seriously injuring Morgan and three others.The passengers in the back of the limo's rear compartment were not wearing seat belts at the time of the crash, NTSB chairman Christopher Hart said at a board meeting, nor had they received a safety briefing outlining the benefits of seat belts.After colliding with the Wal-Mart truck, the limo then struck three other vehicles and rolled over before coming to rest on its side on the New Jersey Turnpike.Because the limo had been customized, "the passengers had no available exits until emergency responders removed parts of a plywood panel that had been installed," Hart said.The driver of the Wal-Mart vehicle, Kevin Roper, was later charged with one count of vehicular homicide and several counts of assault by auto, with authorities saying they believed he had dozed off behind the wheel. Roper pleaded not guilty.In a preliminary report released just weeks after the accident, the NTSB said the trailer was traveling at 65 miles per hour, which is 20 mph over the construction zone's 45 mph speed limit.The crash left Morgan, then 45, in critical condition with multiple broken bones and a traumatic brain injury.Since then, the comedian has struggled to regain his old self, his lawyer said.In a June 2015 interview with "Today" anchor Matt Lauer, Morgan wondered through tears whether he was ever going to be funny again.He filed a civil suit against Wal-Mart, and the two sides reached a confidential settlement in May.According to Morgan's lawyer, the retailer, which initially faulted Morgan and his colleagues for not wearing seat belts, later took full responsibility for the accident.Morgan, a former "Saturday Night Live" and "30 Rock" star, and the others were returning from a show in Dover, Delaware.