The gunman who fired on Las Vegas concertgoers from more than 1,000 feet away on Sunday may have used fully automatic weapons - or guns legally modified to act like them - that allowed him to shoot a hail of bullets into the crowd of 22,000 people.
A US law enforcement official, speaking on condition of anonymity, told Reuters authorities believe 64-year-old Stephen Paddock had at least two such weapons when he fired from his 32nd-floor hotel room, killing at least 59 people and injuring more than 500.
The rat-a-tat sound of the Las Vegas shooter's gunfire prompted police at the scene to report the gunman was using an "automatic" weapon - a term often used to describe a fully automatic gun that can fire as many rounds as its magazine, drum or belt holds by pulling and holding the trigger.
Those weapons have been largely outlawed for three decades, though Paddock could also have used legal or illegal means to alter semi-automatic rifles, which fire a round every time the trigger is pulled.
"From the audio, that is not someone who has a traditional semi-automatic rifle firing it in its normal condition," said David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who advises the gun control group Americans for Responsible Solutions. "Either it's a machine gun or it's been modified. I don't believe a human can do that with his finger."
Clark County Sheriff Joseph Lombardo said investigators found 16 weapons in Paddock's suite at the Mandalay Bay Resort and Casino. He added that officials believe at least some of the guns were modified, possibly to allow for more rapid fire.
It was not clear when or how Paddock obtained the weapons. Chris Sullivan, owner of the Guns & Guitars shop near Paddock's home in Mesquite, Nevada, confirmed Paddock had legally purchased firearms from the store but did not offer more detail.
In 1986, Congress barred civilians from buying or selling fully automatic weapons made after that date, though individuals can legally possess older weapons after passing a background check and obtaining a special permit.
There are about 176,000 pre-1986 machine guns registered with the US government that can be legally transferred, and they typically cost tens of thousands of dollars.
But there are also less expensive legal products that can allow semi-automatic rifles such as the AR-15, which are much more widely available, to fire hundreds of rounds per minute.
A "bump stock," for example, replaces a semi-automatic rifle's stock, which rests against the shoulder to provide stability and absorb recoil.
The bump stock causes the gun's recoil to press against the shooter's finger after each shot, firing rounds much more quickly than possible by pulling it manually.
The website for one such product, Slide Fire, shows several videos in which shooters launch multiple rounds per second in bursts that sound almost indistinguishable from automatic fire. Reviewers have reported that the product permits a shooter to go through hundreds of rounds every minute.
The company did not return requests for comment.
Such devices are not outlawed because the trigger is still pulled for each round, even though the rate is faster than possible using only one's finger.
"There's very little difference between a souped-up semi automatic and a fully automatic," said Mike McLively, a policy expert with Americans for Responsible Solutions.
Paddock could also have illegally converted semi-automatic rifles into fully automatic guns, a process made easier by the presence of guides uploaded to the internet.
Nevada law lets residents own machine guns, if they are permitted under federal law. The state does not require licenses, registration or a waiting period for firearms, including semi-automatic rifles.
Sunday's shooting prompted renewed calls from Democrats for tighter gun laws, though it is unlikely the Republican-controlled Congress will take up such measures.
Shares in gunmakers American Outdoor Brands and Sturm, Ruger & Co rose 3.21 percent and 3.48 percent, respectively, on Monday. Gun stocks often spike following mass shootings on fears of stricter gun control.