Police are seeking clues to explain why a retiree with a penchant for gambling but no criminal record set up a sniper's nest in a high-rise Las Vegas hotel and poured gunfire onto a concert below, slaying dozens of people before killing himself.

The Sunday night shooting spree from a 32nd-floor window of the Mandalay Bay hotel, on the Las Vegas Strip, killed at least 59 people before the gunman turned a weapon on himself. More than 500 were injured, some trampled, in the deadliest mass shooting in modern US history.

The gunman, identified as Stephen Paddock, 64, left no immediate hint of his motive for the arsenal of high-powered weaponry he amassed, including 34 guns, or the carnage he inflicted on a crowd of 22,000 attending an outdoor country music festival.

Paddock was not known to have served in the military, or to have suffered from a history of mental illness or to have registered any inkling of social disaffection, political discontent or radical views on social media.

US officials also discounted a claim of responsibility by Isis.

“We have determined to this point no connection with an international terrorist group,” Aaron Rouse, special agent in charge of the Federal Bureau of Investigation field office in Las Vegas, told reporters on Monday.

Police said they believed Paddock acted alone but were at a loss to explain what might have precipitated it.

“We have no idea what his belief system was,” Clark County Sheriff Joseph Lombardo told reporters. “I can't get into the mind of a psychopath.”

Clark County Sheriff Joe Lombardo briefs members of the media outside Metro Police headquarters (Yasmina Chavez/Las Vegas Sun/AP)

Although police said they had no other suspects, Lombardo said investigators wanted to talk with Paddock's girlfriend and live-in companion, Marilou Danley, who he said was travelling abroad, possibly in Tokyo.

Lombardo also said detectives were “aware of other individuals” who were involved in the sale of weapons Paddock acquired.

Still, the closest Paddock appeared to have ever come to a brush with the law was for a traffic infraction, authorities said.

The death toll, which officials said could rise, surpassed last year's record massacre of 49 people at a gay nightclub in Orlando, Florida, by a gunman who pledged allegiance to Islamic State.

Paddock, however, seemed atypical of the overtly troubled, angry young men who experts said have come to embody the profile of most mass shooters.

Public records point to an itinerant existence across the US West and South East, including stints as an apartment manager and aerospace industry worker. But Paddock appeared to be settling in to a quiet life when he bought a home in a Nevada retirement community a few years ago, about an hour's drive from Las Vegas and the casinos he enjoyed.

His brother, Eric, described Stephen Paddock as financially well-off and an avid enthusiast of video poker games and cruises.

“We're horrified. We're bewildered, and our condolences go out to the victims,” Eric Paddock said in a telephone interview from Orlando, Florida. “We have no idea in the world.”

Las Vegas's casinos, nightclubs and shopping draw some 3.5 million visitors from around the world each year, and the Strip was packed with visitors when the shooting started shortly after 10pm local time on Sunday (4am GMT Monday).

The gunfire erupted during the Route 91 Harvest music festival as country music star Jason Aldean was performing. The musician ran off stage as the shooting progressed.

Video of the attack showed throngs of people screaming in horror and cowering on the open ground, hemmed in by fellow concert-goers, as extended bursts of gunfire strafed the crowd from above from a distance police estimated at more than 500 yards (457 metres). Those at the edges of the crowd fled as best they could, many trampled or hurt jumping over fences while the shooting went on, by some accounts, for about 10 minutes.

The bloodshed ended after police swarming the hotel closed in on the gunman, who shot and wounded a hotel security officer through the door of his two-room suite and then killed himself before police entered, authorities said.

Police said 23 guns were found in Paddock's suite, along with more than 10 suitcases.

Lombardo said a search of the suspect's car turned up a supply of ammonium nitrate, a fertiliser compound that can be formed into explosives and was used in the 1995 Oklahoma City bombing of a federal office building that killed 168 people.

Police found another 19 firearms, some explosives and thousands of rounds of ammunition at Paddock's home in Mesquite, about 90 miles (145km) northeast of Las Vegas, along with some electronic devices, including computers.

They also obtained a warrant to search a second house connected to Paddock in Reno, Nevada, more than 400 miles (644km) northwest of Las Vegas.

Chris Sullivan, the owner of the Guns & Guitars shop in Mesquite, issued a statement confirming that Paddock was a customer who cleared “all necessary background checks and procedures,” and said his business was cooperating with investigators.

“He never gave any indication or reason to believe he was unstable or unfit at any time,” Sullivan said. He did not say how many or the kinds of weapons Paddock purchased there.

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

Lombardo said investigators knew that a gun dealer had come forward to say that he had sold weapons to the suspect, but it was not clear if he was referring to Sullivan. He said police were aware of “some other individuals who were engaged in those transactions,” including at least one in Arizona.

Investigators also want to interview Danley, who police initially described as a “person of interest” but not a suspect. She is an Australian citizen who is reported to have been born in the Philippines.