Las Vegas investigators have given more details about the deadliest shooting in modern US history as they described how the gunman opened fire on airport fuel tanks and on police officers arriving at the massacre.

Clark County Sheriff Joe Lombardo held a news conference alongside the top FBI agent in Las Vegas amid questions about whether police could have done more to stop gunman Stephen Paddock on October 1.

They provided no new information about Paddock’s motivation as he killed 58 people and wounded more than 500 at a country music festival. Forty-five people remain in hospital in a critical condition, Mr Lombardo said.

Sheriff Joseph LombardoClark County Sheriff Joe Lombardo stands by the timeline of events (Erik Verduzco/AP)

The sheriff said a post-mortem has been performed on Paddock, and the coroner observed “no abnormalities” in his brain. He said the brain has been taken to a facility to do a microscopic evaluation.

Mr Lombardo confirmed that Paddock intentionally opened fire on jet fuel tanks at the nearby Las Vegas airport and said he took shots at arriving police officers, possibly to keep them at bay as police rushed to his room.

But nearly two weeks after the massacre, questions remain unanswered.

Map locates shooting in Las Vegas(PA graphic)

Police and the FBI said they are still at a loss to explain his motive but said they have found no signs that he had ideologies or connections to any groups.

In a chronology provided on Monday, Mr Lombardo said Paddock started spraying 200 rounds from his suite into the hallway of the Mandalay Bay at 9.59pm on October 1, wounding an unarmed security guard in the leg.

He said on Friday that the security guard came to a barricaded stairwell door at 9.59pm and was not shot until around 10.05pm.

About that time, the gunman unleashed a barrage of bullets on the festival crowd. Then he killed himself with a gunshot to the head.

Mr Lombardo’s newest version of events aligns with what Mandalay Bay officials said on Thursday. They had disputed whether six minutes actually passed between the first shots in the hallway and the start of the concert rampage and said Paddock may have wounded the security guard within 40 seconds of firing into the crowd.

Earlier this week, lawyers had questioned why police and security were not able to stop Paddock sooner when authorities said six minutes passed between the bursts of gunfire.

Mandalay BayThe Mandalay Bay resort and casino from where Stephen Paddock shot dead 58 people and wounded more than 500 others (John Locher/AP)

Mr Lombardo also pushed back against criticism of his office over whether more could have been done to stop the gunman.

“In the public space, the word incompetent has been brought forward. I am absolutely offended with that characterisation,” he said.

The 10-minute attack on the crowd began at 10.05pm when the 64-year-old real estate investor and retired accountant began firing more than 1,000 rounds from two broken windows. Police did not arrive on the 32nd floor until 10.17pm, two minutes after he had stopped shooting, according to Mr Lombardo.

The wounded guard, Jesus Campos, used his radio to call for help, the statement said. A maintenance worker, Stephen Schuck, has said he also called for help on his radio, asking a dispatcher to call the police because someone was shooting on the 32nd floor.

It is not clear what Mandalay Bay maintenance and security workers did with those messages by the guard and maintenance worker.