Las Vegas police face new questions over their response to last week's deadly mass shooting, after releasing a revised chronology in which the gunman shot a security officer before, not after, opening fire from his high-rise hotel window.

The updated timeline for the bloodiest case of gun violence in recent US history raised new uncertainty over why Stephen Paddock ceased firing on concertgoers once he began, and whether hotel security and police co-ordinated as well as first believed.

Aden Ocampo-Gomez, spokesman of the Las Vegas Metropolitan Police Department, declined to comment on the revised chronology, saying the agency would discuss the implications later.

Paddock, 64, killed 58 people and injured hundreds in a hail of bullets from his suite on the 32nd floor of the Mandalay Bay hotel, overlooking a music festival, and then shot himself to death before police could storm his room.

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Nine days later, his motive remains a mystery.

A Las Vegas police officer stands by a blocked off area near the Mandalay Bay casino in Las Vegas. Photo / AP
A Las Vegas police officer stands by a blocked off area near the Mandalay Bay casino in Las Vegas. Photo / AP

Clark County Sheriff Joseph Lombardo, who oversees the police department, on Monday said Paddock shot a hotel security guard six minutes before beginning to fire on the crowd.

By coincidence, the security officer, Jesus Campos, had been sent to check an open-door alarm on the same floor.

Officials initially said Paddock began raining gunfire onto the concert first, then stopped shooting after strafing the 32nd-floor hallway through the doorway of his room, when Campos was apparently detected via security cameras the gunman had set up outside his suite.

Police officers stand at the scene of a mass shooting near the Mandalay Bay resort and casino on the Las Vegas Strip. Photo / AP
Police officers stand at the scene of a mass shooting near the Mandalay Bay resort and casino on the Las Vegas Strip. Photo / AP

Earlier police accounts said a wounded Campos helped direct police to the room occupied by Paddock, who had quit firing on concertgoers by then.

Lombardo originally said police officers reached the 32nd floor within 12 minutes of the first reports of the attack. That sequence of events was changed in Monday's new timeline issued by Lombardo.

"What we have learned is (the security guard) was encountered by the suspect prior to his shooting to the outside world," Lombardo said.

Lombardo did not address whether the mass shooting could have been prevented, or halted sooner, based on the new chronology, but said it remained unclear why Paddock stopped firing on the concert when he did.

Nevada Lieutenant Governor Mark Hutchison acknowledged to CNN on Tuesday that Paddock did not stop firing because of the guard, Jesus Campos, as had been assumed initially.

Campos immediately alerted the hotel's in-house security team after he was shot at 9:59 pm, six minutes before Paddock first opened fire on the concert, according to Lombardo. But police were not aware Campos had been shot until they met him in the hallway, Lombardo said on Monday.

The sheriff has estimated the time of their rendezvous at 10:18 pm, three minutes after Paddock had stopped firing.

Rather than storm Paddock's suite immediately, police paused to assemble their SWAT team and burst into his room to find him dead 81 minutes after the shooting began, according to the original account.

Leaked photographs from inside the hotel room used by Las Vegas shooter Stephen Paddock in the Mandalay Bay Hotel showing his arsenal of weapons. Photo / Twitter
Leaked photographs from inside the hotel room used by Las Vegas shooter Stephen Paddock in the Mandalay Bay Hotel showing his arsenal of weapons. Photo / Twitter

Police and security officers acted as quickly as possible in the circumstances, said David Hickey, the president of the union that represents Campos, based on what he had heard.

Officials with MGM Resorts International, which owns the Mandalay Bay, questioned the latest chronology from police.

"We cannot be certain about the most recent timeline that has been communicated publicly, and we believe what is currently being expressed may not be accurate," the company said in a statement today.

- AAP