The gunman who killed 59 concertgoers in Las Vegas may have modified his semi-automatic rifles to fire up to 800 rounds per minute and "inflict absolute carnage", officials have said.

Stephen Paddock had two "bump stocks" - devices that allow semi-automatic weapons to fire rapidly and continuously - in his room on the 32nd floor of the Mandalay Bay Hotel and Casino on the Strip.

California senator Dianne Feinstein, who has raised concerns over the sale of such accessories, said on Tuesday that Paddock had modified at least one of his weapons. She said: "Individuals are able to purchase bump fire stocks for less than $200 (£150) and easily convert a semi-automatic weapon into a firearm that can shoot between 400 and 800 rounds per minute and inflict absolute carnage." A fully automatic firearm can unleash continuous rounds until the weapon's magazine is empty.

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Inside room later used by Vegas gunman

In footage of the shooting at the Route 91 Harvest Festival, multiple rapid gunshots could be heard as people tried to flee the scene.

At least 58 people were killed and hundreds of others injured - including at least 45 who remain in a critical condition - following the shooting on Sunday night.

Vegas shooting: Injured total fewer than thought

Vegas gunman's motive remains a mystery

The gunman had smashed two windows in his hotel suite using a hammer shortly after 10pm on Sunday, before aiming bursts of fire at crowds below him.

The former accountant also fired through the door of his room and hit a security guard in the leg. When a SWAT team stormed the room 72 minutes after the first 911 call was made, Paddock was found dead amongst the weapons. He is believed to have shot himself.

On Tuesday, President Donald Trump labelled the gunman a "sick" and "demented" man.

He added: "Lot of problems, I guess, and we're looking into him very, very seriously."

Image: One of the semi-automatic weapons with bullet casings covering the floor. Pic: Fox25Boston

Image: Some of the guns featured scopes and at least one had been modified. Pic: Fox25Boston

Paddock was a multi-millionaire property investor who liked to gamble, and investigators are puzzled over what motivated the 64-year-old to launch the attack. Investigations have centred on his recent financial transactions, after he sent tens of thousands of dollars to someone in the Philippines, where his girlfriend was at the time of the attack, according to ABC News. In the last three years, more than 200 reports about his activities, particularly large transactions at casinos, have been filed with law enforcement, ABC News reported. Some centred around "suspicious activity", but others were "currency transaction reports" which casinos must file with the Treasury Department when someone withdraws or deposits more than $10,000 in cash.

Image: Paddock carried out his attack from the 32nd floor of the Mandalay Bay Hotel

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