Stephen Paddock appeared to be settling into a quiet life two years ago when the wealthy 64-year-old apartment manager and high-stakes gambler bought a home in a rural Nevada retirement community, an hour's drive from his beloved Las Vegas casinos.

Those who knew him say there was no indication he was capable of holing up in a room on the 32nd floor of one of those casinos, the Mandalay Bay, and opening fire on a country music festival across the street.

But Paddock killed 59 people, including himself, and injured more than 500 others on Sunday in the worst mass shooting in modern US history.

Gunman Stephen Paddock broke two windows at the Mandalay Bay Resort and Casino and aimed at the crowd. ( Reuters: Lucy Nicholson )

'He liked video poker and cruises'

The wealthy retiree previously managed an apartment complex called Central Park in Texas, before moving to Nevada. The Washington Post reported he had also worked as an accountant and had real estate investments.

"He was a wealthy guy and he liked to play video poker and he liked to go on cruises," the gunman's brother, Eric Paddock, told reporters from his doorstep in Orlando, Florida, on Monday.

"He's never drawn his gun, it makes no sense."

Sorry, this video has expired Brother of the gunman says there's no logic to explain the shooting

He said he was aware his brother had a couple of handguns he kept in a safe, perhaps a long rifle, but no automatic weapons.

Eric Paddock described the shooter as a peaceful man who moved back to the red desert hills of Nevada partly because gambling is legal in the state and he hated Central Florida's humidity.

The two were last in touch in early September, exchanging text messages about power outages after Hurricane Irma slammed into Florida, where their mother still lives.

"He had nothing to do with any political organisation and religious organisations," Eric Paddock said.

Paddock had a 'god complex'

An executive casino host described the gunman as a very analytical gambler who would try to identify machines most likely to provide big pay-offs.

John Weinreich, a host at the Atlantis Casino Resort and Spa in Reno, said Paddock would sit for hours, placing bets of $100 or more, rarely interacting with anyone but always conscious of his surroundings and who was winning.

Mr Weinreich said Paddock's girlfriend Marilou Danley once worked at the casino and initially got on Paddock's good side by leading him to a machine where someone had lost tens of thousands of dollars.

He said Paddock had a "god complex" and expected quick service, no matter how busy employees were.

"[Paddock liked] everybody to think that he was the guy. He didn't boast about anything he had or anything. It was just his demeanour," Mr Weinreich said.

"It was like, 'I'm here. Don't cross me. Don't look at me too long'."

He was known to stare at people and a high roller told Mr Weinreich she thought Paddock was creepy.

Over a 10-year-period, Paddock worked as a carrier for the US Postal Service, an Internal Revenue Service (IRS) agent and in an auditing department.

A spokeswoman for the Office of Personnel Management said Paddock's employment included about two years as a mail carrier from 1976 to 1978.

After that, he worked as an agent for the IRS for six years until 1984. And then he worked a defence auditing job for about 18 months.

He graduated from college in 1977 from Cal State Northridge and also worked for a defence contractor in the late 1980s.

Gunman was itinerant gambler with pilot licence

In recent weeks, Paddock made gambling transactions worth tens of thousands of dollars, although it was unclear whether they were wins or losses, NBC News reported, citing unidentified law enforcement officials.

Public records point to an itinerant existence across the American West and southeast, having lived in Florida, California and other parts of Nevada.

Paddock had a hunting licence in Texas, where he lived for a while.

He got his pilot licence, and had at least one single-engine aircraft registered in his name.

In early 2015, he bought a modest two-storey home in a new housing development for retirees on the dusty edge of Mesquite, a small desert town popular with golfers and gamblers that straddles Nevada's border with Arizona.

"It's a nice, clean home and nothing out of the ordinary," Mesquite Police Department spokesman Quinn Averett said.

Sorry, this video has expired Video shows chaos outside Las Vegas concert as shooting unfolds.

Police found 19 guns when they raided his home, as well as explosives and several thousand rounds of ammunition.

Roughly an hour's drive southwest is Las Vegas, where Paddock checked into a 32nd-floor room at the Mandalay Bay Resort and Casino last Thursday with at least 23 rifles.

The FBI said he had no connection with international militant groups, however Islamic State claimed responsibility for the massacre.

Paddock killed himself before police entered the hotel room he was firing from, Clark County Sheriff Joseph Lombardo told reporters.

"We have no idea what his belief system was," Mr Lombardo said.

"I can't get into the mind of a psychopath."

Investigators reconstructing Paddock's movements before the shooting have revealed he also booked rooms overlooking the Lollapalooza festival in Chicago in August and the Life Is Beautiful show near the Vegas Strip in late September.

Records as recent as 2015 list Paddock as single, though it appears he may have married while living in California in the 1980s.

Police and public records said he lived with a woman in the Nevada retirement community.

Authorities said she had no connection with the attack.

Stephen Paddock (centre) in the 1970 Francis Polytechnic High School Year Book. ( Supplied: Francis Polytechnic High School )

Paddock's father was on FBI Most Wanted list

Paddock's father was Benjamin Hoskins Paddock, a violent bank robber who was on the FBI's Most Wanted list in the 1960s.

"We didn't know him," Eric Paddock said of their father.

The shooter himself had no criminal record beyond a traffic violation, police in Las Vegas said.