The Las Vegas shooter had 23 weapons, including modified guns

More dramatic mobile phone video is emerging of the terrifying moments when a gunman in the Mandalay Bay Hotel unleashed a hail of gunfire on a crowd enjoying a country music festival in Las Vegas, killing 59 and injuring more than 500 Sunday. (Oct. 3) AP

The Mandalay Bay Resort on Oct. 3, 2017, in Las Vegas. Authorities said Stephen Craig Paddock broke the windows in his rooms there and began firing with a cache of weapons, killing dozens and injuring hundreds. (Photo: John Locher, AP)

The hotel suite used by the Las Vegas gunman to rain deadly rounds down on concertgoers below revealed an alarming arsenal of 23 weapons and accessories, including semiautomatic rifles, scopes and hundreds of rounds of ammunition — much of which could be legally purchased in Nevada. 

Another 19 weapons were found at the shooter's Mesquite, Nev., home.

How the gunman, Stephen Paddock, 64, of Mesquite, acquired the weaponry, how he passed background checks and what his motivations were are all being investigated by authorities. 

Asked about a potential motive, Clark County Sheriff Joseph Lombardo said he could not “get into the mind of a psychopath at this point.”

Photos of the hotel crime scene on the 32nd floor of the Mandalay Bay Hotel and Casino tweeted out Tuesday by Boston 25 News showed what appeared to be a semiautomatic rifle equipped with a bipod and a scope laying on a carpeted floor surrounded by rounds of ammunition. The weapon also appears fitted with an irregular stock. 

An undisclosed number of the weapons at the crime scene are believed to have been modified with technology known as a "bump-stock'' that allows for rapid firing, similar to a machine gun, two people familiar with the matter said.

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The technology relies on recoil energy produced by gun fire to expend rounds in rapid succession and could be readily purchased online or from private dealers, without government regulation. 

The technology was subjected to weeks of review to determine whether it could be classified as an illegal conversion device. Federal authorities determined the technology to be a legal accessory that — when used — did not convert the weapon into an actual machine gun, said Rick Vasquez, a former acting chief of the Firearms Technology Branch at the Bureau of Alcohol Tobacco Firearms and Explosives, who was part of the team that reviewed the bump-stock technology. 

“It did not meet the definition of a machine gun,’’ Vasquez said. “It took a lot of study because ATF is concerned about safety. But sometimes you have to make a decision you don’t necessarily like. It was an accessory, not a conversion device.’’

Semiautomatic weapons, such as the AR-15, could be legally purchased under federal law and have been found at the scene of previous mass killings such as the San Bernardino, Calif., shooting and the Sandy Hook Elementary shooting in Newtown, Conn.

These weapons fire a single shot each time the trigger is pulled, unless the weapon is modified. Some states, such as California and Massachusetts, regulate the sale of these weapons and the size of the magazines that attach to them, but Nevada does not. 

Though Nevada voters expanded gun background checks to private-party sales in a November 2016 ballot initiative, the state's attorney general contends that the checks cannot be put in place because the FBI says it won't handle the checks. (Photo: Getty Images)

Fully automatic weapons fire rounds continuously as long as the trigger is held down. 

Under federal law, machine guns — considered automatic weapons — are tightly regulated but legal to own if they were made before May 1986 and are registered with the federal government.

About 391,000 machine guns were listed in the national firearms registry as of November 2006, according to the Law Center to Prevent Gun Violence, which advocates stricter gun laws. That same year, the Bureau of Alcohol, Tobacco, Firearms and Explosives seized 1,280 unregistered machine guns. 

Unlike Nevada, several states, including California and Massachusetts, ban machine guns outright, with exceptions such as for police training. 

The Sunday night shooting killed at least 59 people attending a country music concert about a block away and wounded 527 others, officials said. Police found Paddock dead in the hotel room, apparently by a self-inflicted wound. 

Paddock's arsenal and the harrowing sound of rapid-fire bursts of gunfire caught on video as he fired volleys at the crowd have renewed a national debate on Americans' relationship with guns and whether any tragedy will prove shocking enough to change it.

Gun shop owners are required under federal law to alert ATF if a client buys more than one handgun from the same store within five days, but the same rule doesn't apply if a gun owner buys mulitple semiautomatic rifles, said David Chipman, a former ATF special agent and senior policy adviser at Americans for Responsible Solutions, which advocates for tougher gun rules. 

That discrepancy allows people like Paddock to stockpile arsenals such as the one used in Sunday's shooting, he said. 

"The amazing loophole here is you could buy two small pistols and ATF will be alerted to that, but you could buy 20 assault rifles and ATF won't be alerted at all," Chipman said. "It doesn’t make any sense."

The lack of regulation around bump stocks — that allows semi-automatic rifles to fire like machine guns — also makes gun regulation less effective, he said. "It's an ingenious workaround of the law," Chipman said. 

Gun owners in Nevada don't need a permit to buy or possess a rifle, shotgun or handgun, according to the National Rifle Association. They can carry a firearm openly in public. Nevadans can also own machine guns or silencers, banned in other states, as long as they're legally registered and within federal compliance. The state does not prohibit possession of assault weapons, 50-caliber rifles or large-capacity ammunition magazines, according to the NRA. 

A law enforcement official, who has been briefed on the matter but is not authorized to comment publicly, said police also found two tripods positioned at the hotel windows in what appeared to be a fully-equipped sniper’s nest to take better aim at the crowd below.

Hundreds of rounds of ammunition were among the suspect’s possessions, a cache that could have sustained him in a much longer assault, the official said.

Authorities believe that the gunman, who had no serious criminal background, purchased many of the weapons legally, though investigators were attempting to determine whether he illegally converted some to operate as fully automatic weapons, the official said.

At least some of his arsenal was purchased legally at Guns & Guitars in Mesquite. 

“He passed every federal background check, every time he bought a gun,’’ owner Janis Sullivan, 67, told USA TODAY.

Contributing: Kevin Johnson, USA TODAY; Associated Press

Follow Jervis on Twitter: @MrRJervis. 

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