Updated at 10:40 a.m. ET, Oct. 4: Since this story was originally published, the Bureau of Alcohol, Tobacco, Firearms and Explosives confirmed that the gunman in the Las Vegas shooting had "bump-fire" stocks in his hotel room. Our original reporting continues below.
While questions remain about the motives of 64-year-old Stephen Paddock, the shooter in the Las Vegas massacre that left at least 58 people dead, law enforcement is evaluating whether his guns were modified so he could more quickly fire rounds.
"We are aware of a device called a bump stock ... that enables an individual to speed up the discharge of ammunition," Clark County Sheriff Joe Lombardo said during a news conference on Tuesday afternoon, though he declined to give more details. The Bureau of Alcohol, Tobacco, Firearms and Explosives is working with the FBI to assess Paddock's weapons.
Authorities say Paddock had at least 23 guns — some with scopes attached — in his room on the 32nd floor of the Mandalay Bay Resort and Casino. Inside his home in Mesquite, Nev., police found an additional 18 firearms and several thousand rounds of ammunition and explosives. California Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, told NPR on Tuesday that an FBI briefing revealed that Paddock had some fully automatic weapons in his hotel room.
Bump stocks are legal and manufactured by various companies. Firing a weapon using a bump stock involves using the power of the recoil when shooting. The bump stock is attached to a weapon, basically replacing the gun's shoulder rest and hand grip. This allows the gun to engage in a sliding, back and forth motion. The shooter keeps the trigger-pulling finger still.
The demand for machine guns
Paul Barrett, deputy director at the Stern Center for Business and Human Rights at New York University, tells Here & Now's Robin Young that he thought the shooter used an automatic weapon when he heard cellphone recordings of the attack. Some estimates say the gunman was able to fire 90 shots in 10 seconds.
"I assumed immediately that it was either an automatic weapon or an AR-15-style weapon that had been customized so that it could be fired the equivalent of an automatic mode," says Barrett, who is also the author of Glock: The Rise of America's Gun.
Automatic weapons are capable of a high volume of fire just by holding down the trigger, while semi-automatic weapons load a shell automatically but only fire one bullet each time you pull the trigger.
Automatic weapons have been heavily regulated since the 1930s because of their use by organized crime lords such as Al Capone.
Later, the Firearm Owners' Protection Act sought to fully crack down on automatic weapons, only allowing for legal ownership of such guns made before 1986 following a strict background check and registration of the gun. More than 490,000 such weapons were registered in a government database as of last year, according to a letter from the ATF.
"It just basically takes money and perseverance to acquire a machine gun because there's a lot of demand," Barrett says. "A lot of gun owners and a lot of collectors like to have a fully automatic machine gun in their collection."
Though it is illegal to mechanically modify a semi-automatic weapon to make it automatic, external devices used to adapt the gun are legal.
"A lot of people like to go out to the range and experience that automatic weapon-firing phenomenon, which is quite awe-inspiring if you've never had the experience of putting that many rounds downrange that quickly," Barrett says.
Rep. Seth Moulton, D-Mass., who protested President Trump's moment of silence on Monday, says regulating gun modification devices is one instance where Congress can take action on gun control.
"This is a great example of a law we could pass that would have had an influence on this tragedy," Moulton tells Young. "It would have made a big difference if he wasn't able to use automatic rifle fire. He would have killed far fewer people."
NPR Digital News intern Jose Olivares contributed to this report.