Anyone who has watched the sickening video clips of Sunday night's Las Vegas mass shooting has heard the sound. It's a staccato crackle of gunfire at a rhythm that almost resembles the cadence of a helicopter's blades, far faster than a human being could repeatedly pull a trigger. That's not the sound of the typical semiautomatic rifle owned by millions of Americans, but of an automatic one—or of a semiautomatic that's been modified to be nearly as deadly.

The shooting at a country musical festival on the Las Vegas strip Sunday night has already become the most lethal in modern American history, with at least 58 people murdered and more than 500 injured by a gunman firing from a window of the Mandalay Bay Resort and Casino. But gun experts who have watched—and heard—recordings of that tragedy note another distinction: It appears to be the first mass shooting in decades to have been carried out with a weapon capable of firing at automatic, or near-automatic, speeds approaching hundreds of rounds a minute.

What remains unknown: How the shooter achieved that rate of fire, given that gun laws make acquiring a fully automatic weapon extremely difficult in the United States, if not impossible. But there's no shortage of potential answers. Exemptions do exist in the 30-year-old automatic weapons ban that make it possible for a civilian to attain one. And more likely, many technical hacks, some legal and some not, enable gun enthusiasts to turn their semiautomatic rifles into deadlier rapid-fire weapons.

"In this country in general, and especially in Nevada, it’s extremely easy for someone like this shooter to amass a giant arsenal of weapons that even without modifications are very dangerous and accurate over great distances," says Mike McLively, an attorney at the Law Center to Prevent Gun Violence. "And then there are all these modifications to amplify their destructive capability, both legal and illegal."

John Sullivan, lead engineer for gun access group Defense Distributed, puts it more simply: "Converting a semiautomatic to fully automatic is very, very easy," he says. "At the end of the day, machine guns are easy to make."

Semi-Legal Upgrades

It's still far from clear what sort of weapon the Las Vegas shooter used, and the Las Vegas Metropolitan Police Department declined to comment to WIRED on what sort of weapons were found in the shooter's room at the Mandalay. But the killer would have had a number of ways to obtain the machine gun or its practical equivalent.

One of the easiest, and a leading theory among gun experts speculating about the shooter's weapons, is a so-called "hellfire trigger," or "gat crank": A simple device bolted to an AR-15's trigger well can allow anyone to fire their semiautomatic by turning a rotary crank, firing several shots with every rotation. That can allow a shooter to easily fire hundreds of rounds a minute, compared to the 80 or 100 shots or so the average shooter could manage with normal trigger squeezes. Though that rate of fire would no doubt overheat a normal AR-15, the shooter reportedly had 19 rifles in his hotel room, potentially making it possible to swap a new one in as needed.

Another easy add-on that transforms a semiautomatic rifle to a virtually automatic one is a so-called "slide fire," or "bump" stock. (Authorities confirmed Tuesday that the shooter had affixed a bump stock to at least one of his rifles.) That device adds a sliding mechanism to the part of the rifle that presses into a shooter's shoulder. The shooter then applies forward pressure on the rifle, so that when the entire rifle is knocked back with the recoil from every detonation of a round of ammunition, that pressure bounces the gun forward again on its sliding mechanism. The shooter merely holds his or her trigger finger in place, and that bouncing pulls the trigger again and again at a rate that approximates machine gun fire, as shown in this slow motion video:

Although gat cranks and bump stocks turn AR-15s and AK-47s into functionally automatic rifles, they're not clearly illegal. While automatic weapons have been banned in the US since 1986, the Bureau of Alcohol, Tobacco and Firearms only considers a gun to be automatic if a single pull of its trigger results in a multiple rounds being fired. The bump stock does in fact pull the rifle's trigger with every shot. The gat crank doesn't involve pulling a trigger at all, leaving it in a more legally murky realm that hasn't stopped the devices from being widely sold.