The man behind the worst mass shooting in modern US history had an arsenal of at least 42 guns, explosives and several thousand rounds of ammo, police said.

But detectives still don't know why the multi-millionaire property investor, who liked to gamble, murdered 59 people attending the country music Route 91 Harvest Festival on the Las Vegas Strip.

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Officers recovered 19 firearms, explosives and ammo from Stephen Paddock's home in Mesquite, Nevada, hours after discovering 23 guns in his room at the Mandalay Bay Hotel and Casino from which he attacked the concertgoers.

Police also found several pounds of ammonium nitrate, a chemical used to make explosives, in the 64-year-old's car.


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Image: The 64-year-old was found dead in his hotel room

Some of the firearms recovered from his hotel room had scopes, while two were reportedly modified to make them fully automatic.

Las Vegas Police Department said officers were searching another home owned by Paddock in Reno, Nevada, adding that authorities were still unclear about the gunman's motive.

"We're hunting down and tracing every single clue that we can get on his background," said Sheriff Joe Lombardo.

"So far, investigators had found no manifesto or anything else to explain Paddock's actions."

He added: "I can't get into the mind of a psychopath at this point."

Detectives have described Paddock as a "lone wolf" and dismissed speculation there were more assailants.

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SWAT teams found the retired accountant dead in his room on the 32nd floor of the hotel.

Paddock's brother said he was "not an avid gun guy" and claimed he had become a millionaire through property investments.

In a televised address, President Donald Trump described the shooting as an "act of pure evil", but did not make reference to a growing row over Americans' right to bear arms.

White House press secretary Sarah Huckabee Sanders later told reporters it was "premature" to discuss tighter gun regulations.

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Mr Trump will visit Las Vegas on Wednesday to "grieve with the friends and family of the victims, offer his support to those recovering from wounds and thank courageous first responders", she added.

City officials have made an urgent appeal for blood donors as around 527 people injured in the shooting are treated in hospitals across Nevada.

Some of those wounded were hit by shrapnel, while others were trampled on as hundreds fled the gig in panic.

"I have no idea who I operated on," said Dr Jay Coates, a surgeon at the University Medical Center of Southern Nevada.

"They were coming in so fast... we were just trying to keep people from dying.

"Every bed was full. We had people in the hallways, people outside and more people coming in."