The suspected Las Vegas shooter attached a "bump-stock" to some of his weapons, which increases the rate of fire on legal semi-automatic weapons and allowing them to function as fully automatic weapons.
Whereas a semi-automatic weapon uses one trigger pull for each round fired, a fully automatic firearm can fire an entire magazine with a single trigger pull.
Stephen Paddock had attached bump stocks to two semi-automatic guns, two officials familiar with the investigation told the Associated Press.
A bump stock takes the place of the gun's shoulder rest with a "support step," which covers the trigger opening. When the shooter pulls the trigger, the recoil causes the gun to "bump" the trigger again.
Automatic weapons are difficult to obtain and purchasers must first pay a large tax and pass a thorough federal check beforehand.
Although the bump stock device is legal, it has received much criticism because it gives the effect of a fully automatic weapon.
Paddock opened fire from the 32nd floor of the Mandalay Bay Hotel Sunday night upon concertgoers at the Route 91 Harvest country music festival. The shooting left 59 dead and over 500 wounded.
A total of 23 guns were found in his hotel room and 19 were found at his home in Mesquite, Nev.The suspect was found dead in his hotel room by law enforcement after the shooting.