Stephen Paddock, who carried out the deadliest mass shooting in U.S. history in Las Vegas, may have used a special attachment called “bump stock” to transform normal fire arms into automatic weaponry.

Following the carnage Sunday which killed at least 59 and injured more than a 500, many wondered how a single shooter could have shot hundreds of rounds into a crowd for more than 15 minutes.

The easy answer would have been that Paddock used automatic guns for his yet unexplained murdering spree; though law enforcement officials are still examining firearms found in the killer's room in Mandalay Bay to determine if they were capable of firing automatically, the presence of a modified bump stock rifle may hold a clue to how a large number of rounds were fired in quick succession.

Since the 1930s, the purchase of fully automatic weapons has been significantly restricted under U.S. laws. When the National Firearms Act was amended in 1986, it further prohibited the transfer or possession of machine guns by civilians.

But a loophole in the laws prohibiting possession of automatic fire arms emerged when bump stocks hit the market.

California Sen. Dianne Feinstein has long railed against the device, telling the Associated Press several years ago, "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," California Sen. Dianne Feinstein, who has long rallied against “bump stocks," the Telegraph reported.

A “bump stock” device replaces the gun's shoulder rest, with a "support step" that covers the trigger opening. The shooter holds the pistol grip with one hand and applies pressure on the barrel with the other. As the shooter's finger comes in contact with the trigger, the recoil causes the gun to slide back and forth, "bumping" the trigger.

Another clue that Paddock may have used a souped-up semi-automatic rather than an automatic weapon is the fact that Guns & Guitars, Inc, a weapons shop in Mesquite, Las Vegas, where the shooter bought most of the weaponry used in the shooting, does not sell automatic guns, the Independent reported.

"We mourn for this tragedy and our thoughts and prayers are with the families of the lost and injured," Christopher M. Sullivan, the general manager of Guns & Guitars, Inc., said in a statement following the shooting, ABC News reported.

"Mr. Paddock was a customer and purchased firearms from our store; however, all necessary background checks and procedures were followed, as required by local, state, and federal law. He never gave any indication or reason to believe he was unstable or unfit at any time. We are currently cooperating with the ongoing investigation by local and federal law enforcement in any way we can.”

Ballistic experts also commented on the fact that the gun used in the mass shooting at the Las Vegas strip may have been modified.

David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who advises the gun control group Americans for Responsible Solutions, said, "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."

Bump stocks are not illegal and are easily available in many U.S. states. Paddock had modified at least two of the weapons used in the massacre with bump stocks.

LVMPD Assistant Sheriff Todd R. Fasulo said that 23 guns were found in the Mandalay Bay Hotel and Casino room, from where Paddock fired into a crowd. An additional 19 weapons were discovered at his home in Mesquite, Nevada.