This story was updated on Monday, October 2 at 11:22 pm

On Sunday night, authorities said a gunman opened fire on a crowd of music-festival attendees from a room on the 32nd floor of the Mandalay Bay Resort and Casino in Las Vegas. The suspect, 64-year-old Stephen Paddock, allegedly shot and killed 58 people in what has become the deadliest mass shooting in American history—firing at a rate that set this incident apart from other recent shootings.

Las Vegas Police said Paddock was found dead when a SWAT team entered his hotel room, where investigators also reportedly found up to 20 firearms. (Las Vegas police have not yet released details of the firearms involved.) Semi-automatic guns shoot only one round for every pull of the trigger. Military-style automatic rifles and machine guns are able to fire multiple rounds per trigger pull.

On Monday evening, ABC News reported that a modified bump stock was recovered from the scene, and that authorities were still examining the weapons to see if any were capable of fully automatic fire. Bump stocks are legal, aftermarket accessories that allow semi-automatic weapons to fire at rates approaching those of fully automatic versions.  

If the shooter used one or more automatic rifles—or even if it’s confirmed that he used legal accessories like a bump stock or trigger crank to approach their rate of fire, as some experts believe audio recordings suggest—it would mark a significant departure from other recent mass shootings.

Thirty years ago, the federal government identified automatic weapons for their unique ability to carry out mass casualty attacks and regulated them differently from other weapons, said Daniel Webster, the director of the Johns Hopkins Center for Gun Policy and Research. “As a result, [they haven’t] been used. Now there’s an exception to that.”

There are currently some 300 million firearms in the United States, though estimates vary. It’s legal, generally speaking, to purchase three types of guns: handguns, shotguns, and rifles. Federal law requires background checks for any transfer of a firearm between federally licensed dealers, but sales between private individuals don’t carry a federal requirement for such checks, although some states impose restrictions. Webster described it as “literally no questions asked.”

If you’re 18 years old, in most states, and aren’t otherwise barred from gun ownership, you can legally purchase a firearm from a gun show or from a seller on the internet. Eighteen states, plus the District of Columbia, have a mechanism to regulate that transaction with a background check, such as a licensing or permitting system. It’s most difficult to buy a gun in states like New Jersey and Massachusetts that have such mechanisms, Webster told me, and easiest to purchase one in states like Montana and Wyoming that do not.

For 10 years, the United States prohibited the manufacture of certain semi-automatic weapons it deemed “assault weapons,” like the AR-15 and AK-47 rifles. But that law expired in 2004. According to The New York Times, most of the firearms used in the 16 most recent mass shootings in the United States were purchased legally and with a federal background check.

Omar Mateen, who murdered 49 people at an Orlando nightclub in June 2016, used at least two legally purchased firearms, a handgun and an AR-15-style rifle. Christopher Harper-Mercer, who killed nine people at Umpqua Community College in Oregon, owned 14 guns, all of which he purchased legally. And Adam Lanza, who killed 26 at Sandy Hook Elementary School in 2012, used two rifles that were legally purchased by his mother.*

America hasn’t recently seen a mass shooting with a fully automatic weapon—which is, Webster believes, “a testament to the effectiveness of regulation.” In 1986, Congress passed a law banning the sale of fully automatic firearms, but automatics that were already in existence and registered before May 19, 1986 were grandfathered in under the law, and remain available for legal purchase by civilians. Essentially, Webster explained, “they weren’t made illegal, they simply said you can’t buy and sell new ones.”

To purchase one, a buyer must submit fingerprints, go through an FBI criminal background check, and pay a $200 tax. The guns themselves are also extremely expensive. Justin Anderson, the director of marketing at Hyatt Guns, a Charlotte-based retailer that bills itself as “America’s Largest Gun Shop,” told me that purchasing a semi-automatic weapon, like an AR-15, would cost you between $400 and $700. But for a fully automatic gun, you might spend $18,000 to $25,000. “There are a group of guys out there who buy full autos and just take them to the range to have fun,” Anderson told me, noting that machine-gun shoots happen regularly all over the country. “I’ve done it myself, it’s a lot of fun.”

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) reports that there were 176,000 of these guns registered in the United States as of February 2016. While it’s still unclear how Paddock obtained his weapons, it’s possible that he could have crafted automatic weapons himself, as a gun owner can convert a semi-automatic rifle to full automatic, if they have the inclination. “It’s a difficult process to do,” Anderson said, as well as “extraordinarily illegal.” You’d need a specific type of bolt, a specific receiver, and a few highly regulated parts. But, he added, “Somebody bent on doing something like this, I’m sure it would not be impossible for him to do it.” He also could have attached a bump stock, or used a trigger crank, a mechanism used to hit the trigger multiple times per second.

If automatic weapons, despite being highly regulated, were used in the attack, it may illustrate the limits of tightening gun regulations in preventing mass shootings. But it’s hard to draw conclusions from a single event because we can’t measure the counterfactual, Webster said.

“What we don’t know is how many mass shootings didn’t happen over the past months and years because people couldn’t get an automatic weapon,” he said. “There are plenty of deranged, angry people out there. If more angry, sociopathic people had had automatic weapons, our numbers would look even worse.”


* This article originally stated that the Sandy Hook Elementary School shooting occurred in 2015. We regret the error.