Two “bump stocks,” designed to enable a semi-automatic weapon to fire like a fully automatic weapon, were found in the hotel room Stephen Paddock used as a perch to perpetrate America’s deadliest mass shooting Sunday night in Las Vegas.

A bump stock dramatically increases a semi-automatic weapon’s rate of fire as the device allows the natural recoil of the gun to “bump” the shooter’s trigger finger, increasing the rate of fire to become nearly indistinguishable from that of a fully-automatic weapon. Since the shooter’s finger is technically pulling the trigger for each round fired, the weapon can legally be classified as semi-automatic.

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The devices, which are legal in Nevada, were found in Paddock’s room on the 32nd floor of the Mandalay Bay Hotel along with 23 firearms, including two scoped rifles mounted on tripods, and hundreds of rounds of ammunition, officials told the Associated Press Tuesday.

The images, obtained by Boston 25 News anchor Jacqui Heinrich, depict two AR-15 rifles, one of which appears to be outfitted with a 100 round high capacity magazine and a “bump stock” retrofit, both of which are available for purchase online and are legal under Nevada law.

The sale and possession of fully automatic weapons has been heavily restricted since the 1930s. In 1986, the federal National Firearms Act was amended, further criminalizing the transfer or possession of machine guns made after the legislation was passed.

Gun control advocates have targeted the devices in the past, arguing they effectively circumvent restrictions against fully automatic weapons through a technicality and absent any meaningful difference between a modified semi-automatic weapon and the more heavily restricted fully-automatic firearms.

California Democratic Sen. Dianne Feinstein expressed concern about the potential consequences of the device’s availability during an interview with AP in 2013.

“This replacement shoulder stock turns a semi-automatic rifle into a weapon that can fire at a rate of 400 to 800 rounds per minute,” Feinstein said.

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Two officials told the AP they are still looking into whether the “bump stocks” were used in the massacre.

David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who advises the gun control group Americans for Responsible Solutions, is confident an unmodified semi-automatic weapon could not have produced the rate of fire captured in audio recordings of the incident.

“From the audio, that is not someone who has a traditional semi-automatic rifle firing it in its normal condition,” Chipman told The Telegraph. “Either it’s a machine gun or it’s been modified. I don’t believe a human can do that with his finger.”

Las Vegas Metropolitan Police Department (LVMPD) Sheriff Joe Lombardo said police found an additional 19 firearms in the shooter’s home as well as “some explosives, and several thousand rounds of ammo.” A search of his car yielded ammonium nitrate, an ingredient in fertilizer frequently used in home made explosives. (RELATED: Las Vegas Shooter Found With The Same Bomb Making Material As The OKC Bombers)

Christopher Sullivan, general manager of Guns & Guitars, a local Mesquite, Nev., gun shop, told The New York Times he sold the gunman two rifles and a handgun within the last year but declined to elaborate further.

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