A veteran Las Vegas police officer who was fired for hesitating in a casino-hotel hallway in October 2017 while a gunman upstairs carried out the deadliest mass shooting in modern US history has been reinstated to his job, authorities said Friday.

Officer Cordell Hendrex is due to return to work March 21 following an arbitrator's ruling in his bid to get his job back, according to the head of the city's police union.

"Nothing coming out of 1 October is a victory," Steve Grammas, Las Vegas Police Protective Association executive director, said of the shooting that left 58 people dead and more than 850 injured.

"But Officer Hendrex is happy that everyone can put this behind them."

Department officials did not immediately respond to a request for comment.

Body-camera video showed Hendrex, a Las Vegas officer since 2007, leading a rookie officer and three Mandalay Bay security officers on the 31st floor of the hotel, one floor below where the gunman was firing and stopping when they heard volleys of gunfire during a three-minute span.

Upstairs, Stephen Paddock, 64, a former accountant and high-stakes video poker player, was using a cache of assault-style weapons he'd amassed to rain bullets for more than 10 minutes from the windows of a 32nd floor suite into a country music festival crowd of 22,000 people.

Most of the rifles were fitted with rapid-fire "bump stock" devices and high-capacity magazines. Paddock killed himself with a handgun before police used explosives to blast through his door and find him more than an hour later.

Hendrex's group remained in the hallway for about five minutes before Hendrex led them halfway up a stairwell toward the 32nd floor.

They remained there for at least 15 minutes, when the video clip ends.

Hendrex later acknowledged in a written police report that he was terrified.

"I froze right there in the middle of the hall for how long, I can't say," he wrote.

Grammas said Hendrex was the only Las Vegas police officer disciplined for inaction directly related to the shooting.

His reinstatement through a union arbitration process was first reported by the Las Vegas Review-Journal.

The arbitrator, "wrote in her opinion that she didn't feel that his actions that evening rose to the level of termination or any discipline at all," Grammas told the newspaper.

Clark County Sheriff Joe Lombardo said last year that some officers received lesser discipline for turning off or failing to activate body-worn video cameras.

Another was disciplined for accidentally firing a three-round burst of gunfire inside the shooter's hotel room.

Grammas noted Friday that Hendrex didn't "run out of the hotel," but became part of the team heading toward Paddock's room.