The gunman who killed 58 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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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.
Paddock had amassed an arsenal of at least 42 guns, explosives and several thousand rounds of ammunition, police said.
He used at least 10 suitcases to smuggle the weapons into his room after checking in on Thursday last week.
Images posted by US media showed automatic weapons attached to tripods lying on the ground of the hotel room, with scores of bullet casings littering the floor around them.
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.
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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."
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.
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Officers recovered 19 firearms, explosives and ammo from Paddock's home in Mesquite, Nevada, hours after discovering 23 guns in his room at the Mandalay Bay Hotel and Casino.
Several pounds of ammonium nitrate, a chemical used to make explosives, were also discovered in his car.
Carolyn Goodman, mayor of Las Vegas, told Sky News: "I think the first thing you have to talk about is how did these horrible people, whether they're terrorists or just a crazed individual, get a hold of weaponry like this?
"And there are so many illegal pathways to do this. You don't have to go through what all of us would do - get a registered firearm, go through the waiting time."
Another home owned by Paddock in Reno, Nevada, is also being searched for clues.