(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.

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, Morgan 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.