Las Vegas mass shooter Stephen Paddock modified his weapons with so-called "bump stocks" — legal weapon accessories that effectively convert them into fully automatic firearms, US officials said on Tuesday.

Investigators found 23 guns in Paddock's hotel room along with 12 of the bump stocks, US Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said.

Investigators found an additional 24 weapons in other locations, including the shooter's home in nearby Mesquite. Paddock bought the rifles, shotguns and pistols in four states - Nevada, Utah, California and Texas, authorities said.

Read more: Motive eludes investigators in Las Vegas massacre:

The bump stocks harness the weapon's recoil to power much faster trigger actuations than normally allowed by legal rifles. The US government approved the device in 2010 on the basis that for each shot fired, the trigger is technically pulled by a finger, keeping the weapon a legal semi-automatic.

'It's so easy'

Over about 10 minutes, Paddock was able to fire countless bullets into a crowd of 22,000, killing 59 of them and injuring hundreds more.

Bump stock manufacturer Slide Force did not respond to news agencies' questions, but the Texas company's Facebook page is filled with videos extolling its features, including one in which a woman gushes, "It's so easy because once you slid it forward and leaned into it, it just fires."

The bump stocks cost as little as $99 (€84) and can enable firing rates of more than 600 rounds per minute. Las Vegas Sheriff Joseph Lombardo confirmed Tuesday that Paddock had at least one of the devices.

"The fact that he had the type of weaponry and amount of weaponry in that room, it was preplanned extensively," the sheriff said, "and I'm pretty sure he evaluated everything that he did and his actions, which is troublesome."

Hidden cameras

 What we know about the Las Vegas shooter Stephen Paddock

Paddock appeared to have heavily premeditated the attack, transforming his 32nd-floor hotel room in a sniper's nest. The 64-year-old even set up two cameras in the hallway hidden on a food trolley and another camera through the peephole of his door to watch for people approaching.

A hotel security guard approached Paddock's room during the rampage and was shot through the door.

Leaked crime scene images circulated on social media showed at least one weapon on a bipod. The images, confirmed as genuine, showed Paddock had stockpiled a large volume of ammunition. Only certain types of ammunition such as armor-piercing bullets are subject to sales restrictions.

Gun control debate

Read more: 'Conservatives must say enough is enough' on US gun control

The revelations of Paddock's extensive arsenal and its apparent legality have sparked a bitter debate over gun control in the US.

"We'll be talking about gun laws as time goes by," President Donald Trump told reporters. Trump's 2016 election campaign was backed by the National Rifle Association, a powerful lobby group.

White House spokeswoman Sarah Sanders said on Monday that Trump was a strong supporter of the Second Amendment of the US Constitution allowing people to keep and bear arms. Trump planned to fly to Las Vegas on a condolence mission.

"I think it's premature to be discussing legislative solutions, if there are any," Republican Senate Majority Leader Mitch McConnell told reporters Tuesday.

Republican representative Chris Collins said gun control was unnecessary, "We are not going to knee-jerk react to every situation."

Australia offers help

Australia has offered its help regarding Paddock's girlfriend Marilou Danley, an Australian citizen.

Paddock, a high-stakes gambler, wired a large sum of money to the Philippines, believed to be intended for Danley, ahead of the shooting.

Danley returned to the US late Tuesday night, escorted by FBI agents. Danley was out of the country at the time of the shooting but authorities hope she can shed light onto Paddock's possible motives.

Authorities were left scratching their head for what motivated Paddock to undertake such a brutal rampage.

Paddock has no known criminal record, no mental illness and no affiliation with any terrorist or other extremist group, despite the so-called "Islamic State" claiming responsibility.

aw/rc (AFP, AP, Reuters, dpa)