Police have made a significant change to the timeline of the Las Vegas mass shooting, saying the gunman shot a hotel security guard before he opened fire on concertgoers.

Clark County Sheriff Joe Lombardo had previously said the guard was shot after gunman Stephen Paddock fired at the country music festival and that the guard’s arrival in the hallway of the Mandalay Bay hotel may have caused Paddock to stop firing.

It was not immediately clear why the timeline of the shooting changed and what the impact could be on the investigation.

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On Monday, Mr Lombardo said security guard Jesus Campos was in a hallway of the Mandalay Bay hotel responding to a report of an open door when he heard drilling from the room of 64-year-old Paddock.

Flowers surround the famous Las Vegas signFlowers, candles and other items surround the famous Las Vegas sign (John Locher/AP)

Paddock, who had installed three cameras to monitor the approach to his suite, opened fire through the door, spraying 200 shots down the hall and wounding the guard, who alerted other security officials, Mr Lombardo said.

A few minutes later, Paddock began a 10-minute massacre that killed 58 people in the deadliest mass shooting in modern US history.

Authorities also said on Monday that Paddock targeted aviation fuel tanks, stocked his car with explosives and had personal protection gear as part of an escape plan.

Paddock had power tools and was attempting to drill a hole in an adjacent wall, perhaps to mount another camera or to point a rifle through, but he never completed the work, Mr Lombardo said.

A woman criesAlexandra Gurr cries as she lays flowers for the dead (John Locher/AP)

He also drilled holes and bolted a metal bar to try to prevent the opening of an emergency exit staircase door near the door of his room.

Mr Lombardo again expressed frustration with the pace of the investigation, but not with the investigators who have yet to pinpoint the motive behind the shooter’s decision to fire from the 32nd floor of the Mandalay Bay hotel casino on a Las Vegas Strip concert crowd of 22,000 on October 1.

“It’s because this individual purposely hid his actions leading up to this event, and it is difficult for us to find the answers to those actions,” Mr Lombardo said.

“We believe he decided to take the lives he did and he had a very purposeful plan that he carried out.”

There is still no evidence Paddock was motivated by ideology, or that there was another shooter, he said.

People huddled in prayerPeople pray at a makeshift memorial for Stephen Paddock’s victims (John Locher/AP)

Investigators have found 200 incidents of Paddock moving through the city, and at no time was he with anyone else, Mr Lombardo said.

Meanwhile, friends and relatives of the victims and other concertgoers who survived returned to the scene on Monday to reclaim prams, shoes, phones, backpacks and purses left behind in the panic as they fled.

The personal effects being recovered were strewn across the massive grassy concert venue where country music fans attended the Route 91 Harvest festival have become sentimental memories of loved ones for some and haunting reminders of the night of terror for others.