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 semi-automatic rifle owned by millions of Americans, but of an automatic one—or of a semi-automatic 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 semi-automatic 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, the lead engineer for gun access group Defense Distributed, puts it more simply: "Converting a semi-automatic 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.