WATCH:British tourist describes moment gunfire erupted at Las Vegas concert

She was among those who escaped as the big-spending gambler who took 23 guns into his hotel room before opening fire on crowds attending an outdoor music festival.

Paddock sprayed bullets on revellers enjoying the Route 91 Harvest Festival from the 32nd floor of the Mandalay Bay Hotel and Casino on Las Vegas Strip.

The 64-year-old retired accountant from Mesquite, Nevada, killed himself before officers stormed Room 135 in the gold-coloured glass skyscraper.

Assistant sheriff Todd Fasulo said Paddock had 23 guns - some with scopes - in his hotel room.

Authorities found 19 more guns at his home in Mesquite, as well as explosives and thousands of rounds of ammunition.

Several pounds of ammonium nitrate, a fertiliser that can be turned into explosives such as those used in the 1995 Oklahoma bombing, were discovered in his car.

Investigators believe the shooting - the most deadly in modern US history - was a "lone wolf" attack, and say they do not have any information to lead them to suspect there were more assailants.

Paddock's brother described the shooter as a multi-millionaire property developer who liked to gamble large amounts of money at the world-famous casinos in Las Vegas who had no known political or religious agenda.

Eric Paddock told reporters his brother often received free rooms and meals from casinos, telling reporters: "He was a guy who had money.

"He went on cruises and gambled."

He told the Orlando Sentinel: "We are completely dumbfounded. We can't understand what happened."

He also said their father was a bank robber who was once on the most wanted list.

Investigators are working to establish a motive for the attack, but a law enforcement official said there was no immediate indication that the massacre was connected to international terrorism.

The Islamic State group claimed responsibility for the attack and said the gunman was "a soldier" who had converted to Islam months ago, but it provided no evidence to back up the claim.

Country music star Jason Aldean was performing on Sunday night at the end of the three-day festival in front of a crowd of more than 22,000 when the gunman opened fire from inside the 44-floor hotel across the street.

Paddock had checked into the hotel room on Thursday, authorities said.

People attending the country music concert described scenes of horror as they realised that what they first thought were fireworks were actually gunshots.

Harrowing video footage showed Aldean stopping his performance after an initial volley of shots could be heard.

After a pause, the gunman fired another volley, with the muzzle flashes visible from the casino as victims fell to the ground, while others fled in panic.

Jason Sorenson, of Newport Beach, California, said he first realised something was wrong when musicians left the stage.

He ran and said "we saw people with blood all over their shirts".

Brandon Clack, of La Palma, California, said he heard many shots fired and the shooting that "went on for a long time, like 10 minutes".

Concertgoers fled into casinos and crammed into cars to get away from the shooting.

Barbara Magro, of Orange County, California, said "they were getting into people's trucks. I saw one with about 20 people in the back".

Police shut down busy Las Vegas Boulevard, and federal and state authorities converged on the scene.

Interstate 15 was briefly closed, and flights at McCarran International Airport were suspended.

Hospital emergency rooms were jammed with the wounded.

The dead included at least three off-duty police officers from various departments who were attending the concert, authorities said.

Two on-duty officers were wounded, one critically, police said.

Assistant sheriff Fasulo said Paddock shot a security guard on the 32nd floor of the Mandalay Bay Hotel.

In an address to the country, US president Donald Trump called the attack "an act of pure evil" and added: "In moments of tragedy and horror, America comes together as one. And it always has."

Prime Minister Theresa May wrote on Twitter: "The UK's thoughts are with the victims and the emergency services responding to the appalling attack in Las Vegas."

Before Sunday's massacre, the deadliest mass shooting in modern US history took place in June 2016, when a gunman who professed support for Muslim extremist groups opened fire at a gay nightclub in Orlando, Florida, killing 49 people.