The man who carried out the deadliest mass shooting in modern US history was a “lone wolf” – a divorced accountant who lived in a retirement community outside of Las Vegas and whose murderous actions have left both police and the people who knew him scrambling to find a motive.

Officials said 64-year-old Stephen Paddock shot himself some time before a police Swat team stormed his room on the 32nd floor of the Mandalay Bay hotel-casino, from where he had opened fire on a country music festival where 22,000 people were present, armed with more than 10 rifles and two tripods.

Officials believe he had altered those legally purchased weapons to operate on automatic, before he began his deadly spree at around 10.08pm on Sunday.

Paddock had checked into the hotel on Las Vegas’s celebrated Strip three days earlier, having driven from his home in the city of Mesquite, 80 miles to the north-east, where he lived with his girlfriend.

Police had initially thought that the woman, Marilou Danley, was somehow involved in the attack, but detectives now believe Paddock used her ID to check into the room. She is currently outside of the US, in the Philippines, and police have spoken to her.

As police began to put together a profile of Paddock – he had moved from Florida two years ago, he had bought the modest property in 2015, previously he had lived in Texas, California, and other places in the West, he enjoyed $100-a-hand poker – they said they were struggling to answer the most pressing question: what drove him to perpetrate such a deadly, calculated act.

“Right now, we believe it’s a sole actor, a lone-wolf-type actor,” Las Vegas sheriff Joseph Lombardo told reporters, saying he was a “psychopath”.

Las Vegas shooting: Police radio communication during the attack is released

“We have no idea what his belief system was. Right now, we believe he was the sole aggressor.”

The city’s mayor, Carolyn Goodman, called him a “crazed lunatic full of hate”, but officials also said background checks had failed to show up any “derogatory” information and they were quick to dismiss any links to international terror groups, seemingly debunking a claim of responsibility by Isis.

Paddock’s brother could similarly offer few insights. Eric Paddock told Reuters the family was stunned by the news and that his brother had not been an “avid gun guy”.

“We have no idea. We’re horrified. We’re bewildered and our condolences go out to the victims,” Mr Paddock said. “We have no idea in the world.”

He said he had helped his brother move to Nevada two years ago in order for him to be able to play more poker and video poker. The two were last in touch a few weeks ago, when they exchanged text messages about power outages after Hurricane Irma hit Florida.

The Orlando Sentinel said a former neighbour, Sharon Judy of Viera, Florida, said Paddock was a friendly man who had described himself as a professional gambler. Once he had showed Ms Judy a picture of himself winning a $20,000 slot-machine jackpot. “He was friendly all the time,” she said.

As part of their search for clues, police are searching Paddock’s home in Mesquite, where they discovered some guns and ammunition, though not in large numbers. Property records show the home is located in a retirement community, Sun City Mesquite, a development of about 1400 homes. The community includes an 18-hole golf course, indoor and outdoor swimming pools and a recreation centre. Reports said residents must be at least 55 years old, and no children are allowed.

Las Vegas shooting – in pictures Show all 15 1 / 15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

Police said officers had located a Hyundai Tucson and a Chrysler Pacifica Touring, both with Nevada license plates, which were registered to the dead suspect.

“We don’t have a lot on Mr Paddock. He does live here in our city. We have not had any law enforcement contact with him in the past,” Quinn Averett, a spokesman for the Mesquite police department, said at a press briefing.

“It’s a nice, clean home and nothing out of the ordinary.”

One detail of Paddock’s profile that leapt out in media reports was revealed by his brother – namely that his father was a well-known bank robber and at one point was on the FBI’s 10 most wanted list. Their father, Patrick Paddock, died a few years ago.

CNN said Paddock had bought multiple firearms in the past, several of them purchased in California. Yet it does not appear that those weapons were among the ones used in the Las Vegas massacre, it said.