A gunman accused of carrying out the worst mass shooting in modern US history had an arsenal of at least 42 weapons, US police have said, as they scour for clues to determine the motive behind the attack in Las Vegas.

Perched on the 32nd floor of the Mandalay Bay hotel, Stephen Paddock opened fire from his hotel room - a veritable sniper's nest - onto a crowd of thousands of concert-goers, killing at least 59 people and wounding 527 others.

The 64-year old former accountant was later found dead in his hotel room, and police believe he killed himself.

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"We have recovered 23 firearms at Mandalay Bay and 19 firearms at his home in Mesquite," Todd Fasulo, assistant sheriff, said on Monday evening, a day after the shooting spree. Mesquite is about 120km northeast of Las Vegas in the state of Nevada.

"The Las Vegas metropolitan police department is working on two fronts to process the crime scene and also investigate the motive," Fasulo said.

"We're hunting down and tracing down every single clue we can get in his background," he added.

'It was really scary'

Paddock fired off hundreds of bullets from a fully automatic, military-style machine gun on a crowd of more than 22,000 people attending an open-air music festival while country-music star Jason Aldean was performing.

Among them was Sarah Haas.

"Boom boom boom, and then one after the other," she said, describing the moment of the attack.

"We were laying down on the floor; I didn't know whether to get up, to run, to stay, to duck. I didn't know if it was safe to move because of everything that was going on. It was really scary."

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Al Jazeera's Andy Gallacher, reporting from Las Vegas, said that some of the weapons that Paddock "had legally bought had been modified to go from semi-automatic to automatic".

This meant that the gunman "could let off between 400 and 800 rounds every minute", he said.

The attacker apparently had no criminal record, no history of mental illness, and the FBI said he had no connection with overseas armed groups.

Eric Paddock, Stephen's brother, said on CNN that his brother had "absolutely" no mental problems or political motives that the family was aware of.

"I think this will be a very long investigation, as this is a man who had very little contact with the police, never really got in trouble and lived a pretty quiet life in Mesquite," our correspondent said.

'Eerily quiet'

A day after the attack, residents and community leaders around Las Vegas held multiple prayer vigils to honour the victims of the attacks and first responders.

"This is very clearly a city in mourning," Al Jazeera's Gallacher said.

Reporting from the site of the attack just after midnight on Tuesday, he said that Las Vegas was "eerily quiet".

"Normally this whole area would be full of partygoers, revellers people who've come to enjoy what is known as the entertainment capital of the US.

"But right now, all I can see around me are abandoned shoes, and there's actually blood on the sidewalk as people escaped from that concert."

READ MORE: Timeline - Twenty years of mass shootings in the US

Sunday's attack is the deadliest US mass shooting in modern history.

So far in 2017, the watchdog group Gun Violence Archive has documented 273 mass shootings in the US.

The group also recorded 11,621 gun-related deaths and 23,433 firearm-related injuries during that period.

The Mandalay Bay attack comes just weeks after Spencer Hight carried out a mass shooting during a gathering at his estranged wife's home in Plano, Texas.

Hight killed eight people and was later shot dead by police.