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.
The board said Roper declined to speak to its investigators.
But the board determined that Roper was awake for more than 28 consecutive hours before the crash, having driven 12 hours in his personal vehicle before he started his job on the morning of June 6, the NTSB said.
"Drivers have a professional responsibility to do the right thing," NTSB member Robert Sumwalt said Tuesday. "Their professional obligation (is to their company) and the people who share the road with them.
"This driver abdicated each of those responsibilities."
Walmart, which earlier this year settled lawsuits by McNair's family
and other injured passengers for undisclosed amounts, has said Roper did not break any rules.
The NTSB released the probable cause and made nine new safety recommendations after a Tuesday morning board meeting in Washington, finishing months of investigation.
Trucker drove 800 miles to work, then began shift, NTSB says
Roper began his waking period on the night of June 5, and drove 800 miles in his own vehicle from his home in Georgia to a Walmart facility in Smyrna, Delaware, where he logged in for duty at 11:22 a.m. June 6, the NTSB said.
He made deliveries and pickups throughout the day and was making his final delivery for his shift when the wreck happened about 12:54 a.m. June 7, about a half-hour short of his 14-hour duty limit, the NTSB said.
Driving 12 hours before his work even began played a factor in the crash, causing him to be fatigued to the point where he didn't realize in time that he needed to brake for the slowed limo van, an investigator told the board.
Had Roper been traveling the posted speed limit of 45 mph rather than 65 mph, and had he braked at the same point, the crash wouldn't have happened, an investigator told the board.
After the crash, Walmart began requiring drivers to live within 250 miles of the place they report to work, or be at that starting point at least nine hours before their shift begins, the NTSB said.
Walmart also has said it would develop a "fatigue management program" that would educate drivers, dispatchers, managers and their families about the need to report to duty refreshed.
Among its nine recommendations, the NTSB called on Walmart to finish implementing the program, and it wants the Federal Motor Carrier Safety Administration to require such measures nationwide, NTSB Chairman Christopher Hart said in his closing statement Tuesday.
"Strong hours-of-service rules are important, but they cannot govern what employees do on their own time," Hart said.
NTSB: Passengers didn't wear seat belts
Of the seven people in the limo van, only one was wearing a seat belt even though New Jersey requires all occupants to be buckled, the NTSB said.
The board said the National Limousine Association should urge limo drivers to give their passengers pretrip safety briefings and urge them to wear restraints.
Among the NTSB's other findings and recommendations:
• Confusion and inexperience among emergency responders led to poor triage and a delay in delivery of patients to a trauma center, which was 16 miles away. One victim who should have been immobilzed was moved without being secured to a backboard. The NTSB recommended that New Jersey health officials establish minimum training and practice standards fof organizations that provide emergency services on the New Jersey Turnpike.
• The van's passenger compartment door was jammed in the crash, and rescuers had trouble reaching the injured occupants. The National Highway Traffic Safety Administration should require that limo vans' passenger compartments have at least two exits, the NTSB said.
Roper was charged last year with vehicular homicide and assault by automobile. He pleaded not guilty.
Morgan was in a coma after the accident and suffered broken ribs, a broken nose, a broken leg and brain trauma.
In June, Morgan told NBC's "Today" show
that he did not remember the crash and still had not been able to return to work but hoped to do so soon.