Authorities say the Las Vegas shooter put a camera inside the peephole of his hotel room to see down the hallway as he opened fire on a crowd of concertgoers.

Undersheriff Kevin McMahill told reporters on Tuesday (US time) that Stephen Paddock also set up two cameras in the hallway outside his room at the Mandalay Bay casino-hotel so he could watch law enforcement or security approach.

He says Paddock fired on and off for nine to 11 minutes and unleashed a dozen or so volleys. He says the first call about shots fired came in at 10.08pm Sunday and the gunfire stopped at 10.19pm.

Authorities also released police body camera video that showed the chaos of the Las Vegas shooting as officers tried to figure out the location of the gunman and shuttle people to safety.

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Amid sirens and volleys of gunfire, people yelled, "They're shooting right at us!" while officers shouted, "Go that way!" during the Sunday night attack, which killed 59 people and left more than 500 injured.

Federal officials say the Las Vegas shooter had devices attached to 12 weapons that allow semiautomatic rifles to mimic fully automatic gunfire.

Bureau of Alcohol, Tobacco and Firearms Special Agent in Charge Jill Schneider also told reporters Tuesday that Stephen Paddock had nearly 50 guns in three locations.

She said he had a combination of rifles, shotguns and pistols.The gun attachment that mimics automatic gunfire is a little-known device called a "bump stock" that was not widely sold.

The stocks have been around for less than a decade, and Schneider said officials determined they were legal.

Investigators work at a festival grounds across the street from the Mandalay Bay Resort and Casino. Photo / AP
Investigators work at a festival grounds across the street from the Mandalay Bay Resort and Casino. Photo / AP

A killer's deadly plan revealed

On Thursday, September 28, Stephen Paddock checked in to room 32135 on the 32nd floor of the Mandalay Bay Resort and Casino in Las Vegas.

The 64-year-old high-roller, who lived just an hour from the gambling mecca, had the perfect vantage point over the Route 91 Harvest festival, which began on Friday.

Paddock had scored some big wins over the previous few weeks, pocketing more than $10,000 on several occasions and boasting of one $250,000 jackpot. But for some reason, with his girlfriend away in Tokyo, his thoughts had turned to murder.

It's believed he had rented several apartments in Downtown Vegas the weekend before, overlooking another festival headlined by Lorde, but it was on Sunday that he was to carry out the deadliest shootings in US history.

Between Thursday and Sunday, the millionaire retiree brought 22 guns up to his double-room suite, stashed in 10 suitcases. Two were on tripods and two converted from semi-automatic into fully automatic machine guns.

In his car, parked with the hotel valet, were several kilograms of ammonium nitrate, a fertiliser used to make explosives, the New York Times reported.

Back at his home at a retirement community in Mesquite, Nevada, were a further 19 guns, several thousand rounds of ammunition and a different explosive, tannerite. All his weapons were obtained legally.

The twisted genius of Paddock's deadly plan was that he did not need to bypass security at the country music festival. Instead, on Sunday night at around 10.08pm, he smashed two windows in his suite with a hammer, positioned his guns and unleashed a hail of gunfire of the crowd gathered on Las Vegas Boulevard just 450 metres away.

Singer Jason Aldean stopped playing and ran from the stage as screaming fans dropped to the ground or frantically tried to escape being mowed down.

But Paddock's form of attack was "like shooting fish in a barrel", one witness told CNN, with his aerial outlook allowing him to pick off his victims, killing 59 and leaving 520 injured.

Las Vegas Metropolitan Police Department Sheriff Joseph Lombardo explained the room was one large suite with uninterrupted views of the Las Vegas Strip, meaning he had a clear view of the festival.

The video below illustrates the views Paddock had from the hotel room.

As Paddock fired on and off for 15 minutes, hotel guests on the 29th to 32nd floor reported hearing glass smashing to management. It was 72 minutes after the first shot that hotel security and a SWAT team break down his door, thanks to a smoke alarm.

It is believed Paddock turned the gun on himself at that moment. He was found sprawled among his guns on the plush hotel room carpet, the heavy curtains slightly parted.

Why gunman's methods are so terrifying

Like the bombers who targeted the Ariana Grande concert from the Manchester Arena foyer in May, Paddock managed to kill from a position where he didn't need to pass through the security checks or metal detectors at the concert.

His evasion of hotel security and choice of an open-air target has raised major problems for security at future events.

Do hotels need to start X-raying guests?

The Austin City Limits music festival, which is due to be attended by 75,000 people a day in a Texas park from Friday to Sunday, is ramping up security amid fears of a copycat attack.

Officers have visited apartments that overlook the festival grounds on the north side of the park and organisers C3 Presents released a statement describing a "layered security plan that includes elements that are seen and unseen", that will include "an enhanced security and law enforcement presence inside and outside the festival."

Nevertheless, they have offered refunds to fans who no longer want to attend.

Austin Interim Police Chief Brian Manley said there would be extra police presence at the event headlined by Jay Z and the Red Hot Chili Peppers, but warned: "We live in a world now where you cannot protect against every single threat."

Drapes billow out of broken windows of the room at the Mandalay Bay resort and casino where Stephen Paddock shot at concertgoers. Photo / AP
Drapes billow out of broken windows of the room at the Mandalay Bay resort and casino where Stephen Paddock shot at concertgoers. Photo / AP

Why did he do it?

As for what may have set Paddock off, retired FBI profiler Jim Clemente speculated there was "some sort of major trigger in his life - a great loss, a break-up, or maybe he just found out he has a terminal disease".

Clemente said a "psychological autopsy" may be necessary to try to establish the motive. If the suicide didn't destroy Paddock's brain, experts may even find a neurological disorder or malformation, he said.

He said there could be a genetic component to the slaughter: Paddock's father was a bank robber who was on the FBI's most-wanted list in the 1960s and was diagnosed a psychopath.

"The genetics load the gun, personality and psychology aim it, and experiences pull the trigger, typically," Clemente said.

- AP, news.com.au