Las Vegas sniper Stephen Paddock left no stone unturned before orchestrating the deadliest mass shooting in US history.

More details have emerged on the extraordinary planning the 64-year-old went to before he opened fire on a 22,000-strong crowd from the 32nd floor of the Mandalay Bay Resort and Casino. At least 58 people were killed during the rampage.

Authorities said Paddock "extensively" planned the attack after discovering he set up cameras around the hotel to spy on police, even hiding one in a food service cart.

But now, a Wall Street Journal report reveals Paddock may have also had "target practice" in the desert near his home two days before the massacre.

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Investigators have reportedly uncovered video footage from a home-surveillance system that shows Stephen Paddock driving alone to an area on the outskirts of Mesquite, Nevada, where locals go for target practice.

The footage shows Paddock heading to the area on the Friday before the Sunday attack, the official told the Journal. It was one spot where he reportedly practised shooting.

The video footage shows Paddock driving along a road that leads to a spot in the desert where everyone goes to shoot, the law-enforcement official added.

However, a range manager told the Journal Paddock had no military service record and never used his arsenal at the only shooting range within 32km of his home.

A spokeswoman for the Las Vegas Metropolitan Police Department didn't respond to a request for comment.

Cops describe 12 minutes of hell

Las Vegas police officers described in harrowing detail the tense 12 minutes they spent trying to breach the mass murderer's lair - and the paranoia that gripped them once they were inside, the New York Post reports.

Smoke hung thick inside the room - tinged with the scent of spent gunpowder and sporadically illuminated by a blinking fire-alarm light - when officers first stepped into Mandalay Bay's Room 32-135, where Paddock, now dead from a self-inflicted gunshot wound, spent the previous three days building his sniper's nest, according to the police in an interview with CBS TV's 60 Minutes.

The sight - and the seeming impossibility that a lone man in his 60s could have caused such carnage - only intensified the officers' paranoia as they worked to clear the hazy hotel room, they said.

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Officer Dave Newton responded to the Vegas shooting. Photo / CBS News
Officer Dave Newton responded to the Vegas shooting. Photo / CBS News

"It was still very much in my brain there's 50 other dudes in here somewhere," Detective Matthew Donaldson recalled. "You know, we were still clearing that room, the curtains, moving the curtains. I wanted to make sure somebody wasn't hiding between the windows and the curtains."

They had reason to fear Paddock had help, though investigators later determined he acted alone: The first person to reach the room, Mandalay Bay security guard Jesus Campos, was met with a hail of gunfire when he had approached the door just minutes before.

By the time Paddock was done shooting at Campos, who escaped with a leg injury, the door to his room looked like "Swiss cheese," Officer David Newton told 60 Minutes.

Newton and other officers coming from the floor below struggled to open a stairwell door that Paddock had jammed shut with metal shims and screws.

Then they saw his splintered door room and a nearby room-service cart with wires hanging off it, prompting fears it was booby-trapped. It was later revealed that Paddock had installed a camera on the cart to watch for anyone coming to stop him.

As officers prepared to blast open what was left of Paddock's door, they worried that the slightest noise could give away their positions - and lead to another volley of gunfire from the room.

They eventually blasted open the door, revealing "an armory" inside, according to Sgt. Joshua Bitsko.

Broken windows from the room where Paddock carried out his deadly shooting. Photo / AP
Broken windows from the room where Paddock carried out his deadly shooting. Photo / AP

Along with hundreds of spent casings, officers found Paddock's lifeless body on the floor. As cops stepped over the corpse and peered through smoke, the totality of Paddock's sick project came into focus.

"Days of planning," Bitsko said. "He had toolboxes, power tools to run wires for his surveillance systems. For everything that he had, it took him days to finish."

Seeing the level of preparation only innervated police as they scoured the suite and adjoining room for any sign of accomplices.

When they found no one else, they left as quickly as they came.

"And then - once that scene was static it was - it was essentially a crime scene. It was, like, 'Stop what you're doing. Get out,'" Donaldson said.

Paddock: 'I was born bad'

The Sun reports Paddock boasted in text messages about his father Benjamin being a bank robber who escaped jail in 1969.

He wrote: "I didn't have anything really to do with him but the bad streak is in my blood. I was born bad."

The escort described him as "obsessive" and "paranoid" and said he would rant about conspiracy theories, including claiming 9/11 was an inside job by the US government.

Inside the gunman's hotel room on the 32nd floor of the Mandalay Bay Resort and Casino. Photo / Twitter
Inside the gunman's hotel room on the 32nd floor of the Mandalay Bay Resort and Casino. Photo / Twitter

The woman, 27, saw Paddock around nine times from November 2015 to June 2016 when he would visit Las Vegas without his girlfriend Marilou Danley, 62.

Paddock - who made $5.8 million in 2015, mostly from gambling - would pay her up to $10,000 a time.

They even stayed in the Mandalay Bay, from where he was to open fire on a music festival, slaughtering 58 before killing himself.

The escort, who does not want to be named, said: "When I first met Stephen I had left an abusive relationship and was starting all over again with nothing but the clothes I was wearing.

"He seemed like he wanted to help. We would go to the casinos together and he would spend hours drinking and gambling.

Paddock's former girlfriend Marilou Danley. Photo / Las Vegas Metropolitan Police Department
Paddock's former girlfriend Marilou Danley. Photo / Las Vegas Metropolitan Police Department

"But when he would have a winning streak, we would go back and have really aggressive and violent sex."

The Sun was reportedly shown text messages which appeared to show Paddock discussing tying her up "as you scream for help".

The escort broke off contact when she started a new relationship.

She said: "He had a dark and twisted side. But even so, I could never have imagined he would do something like this."

Gunman had 'vanilla profile'

The owner of a casino where Stephen Paddock gambled says the Las Vegas gunman didn't drink alcohol or show any signs of trouble when he was a customer there. Steve Wynn said in an interview on "Fox News Sunday" that Paddock had "the most vanilla profile one could possibly imagine."

He says Paddock didn't run up debts or have a gambling problem. Wynn said butlers, waiters and massage employees knew Paddock and his girlfriend.

Investigators walk through debris on festival grounds. Photo / AP
Investigators walk through debris on festival grounds. Photo / AP

Wynn also described some of the security measures his casino put in place in recent years that include magnetometers and training of housekeeping staff to report suspicious actions like a do-not-disturb sign remaining on a door for an extended period of time.

FBI returns lost belongings

Officials say they're beginning to return personal belongings to concertgoers who were in one corner of the festival grounds.

Deputy Clark County Fire Chief John Steinbeck announced that items are being returned to people at a Family Assistance Center in the South Hall of the Las Vegas Convention Center.