The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said that the man behind a recent mass shooting in Las Vegas modified 12 guns to have a faster firing rate, according to ABC 15 Arizona.
ABC 15 on Tuesday reported that ATF officials confirmed Stephen Paddock, 64, augmented the weapons with bump-fire stocks to make the semi-automatic weapons imitate automatic gunfire.
ATF spokeswoman Jill Snyder said that the devices are not illegal to own as they do not mechanically alter the weapon, adding that Paddock had pistols, rifles and shotguns during last Sunday's massacre.
Paddock continuously fired on an outdoor concert for nine to 11 minutes during the bloodshed, according to the ATF, killing at least 59 people and wounding at least 527 others.
The gunman behind the worst mass shooting in modern U.S. history had two devices he used to speed the rate of his firing.
Officials said that Stephen Paddock, 64, possessed a pair of “bump-stocks” that could have converted his semi-automatic weapons into fully automatic ones.
Semi-automatic firearms require one trigger pull for every round fired, while their fully automatic counterparts can use a single pull to unleash continuous rounds until a clip empties.
Two officials familiar with the investigation told The Associated Press that he had two bump stocks.
AP’s source said authorities are probing whether those stocks were used to modify guns used in last Sunday’s bloodshed.
A bump stock replaces a gun’s shoulder rest, with a “support step” that covers the weapon’s trigger opening.
A shooter can then hold the pistol grip with one hand and push forward on the barrel with the other, while their finger comes in contact with the trigger.
This recoil causes the weapon to rock back and forth while firing, “bumping” the trigger with the gun user’s finger.
The weapon subsequently remains a legal semi-automatic as technically an individual’s finger is pulling the trigger for each round fired.
Some Twitter users on Tuesday blasted the lack of gun control measures regarding bump stock devices.
Paddock killed at least 59 people and wounded about 527 others when he opened fire last Sunday on an outdoor country music event.
The gunman was located in a 32nd floor hotel room of the nearby Mandalay Bay Resort and Casino when the carnage began.
Police officers who ultimately stormed Paddock’s room found him dead inside it with 23 firearms.
The incident has reignited fierce national debate over America’s gun control laws, with Democrats pushing for tougher measures following the shooting spree.
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The Associated Press contributed to this report.