Investigators in Nevada are looking into how the Las Vegas shooter was able to fire his weapons so quickly, and some reports point to relatively inexpensive – and legal – rifle accessories.

Twelve "bump stocks" were found in Stephen Paddock’s Mandalay Bay hotel room, according to an Associated Press report. Similar devices are sold in East Tennessee.

Bump stocks replaced a semi-automatic rifle’s factory stock. They use the recoil of the weapon to repeatedly "bump" the trigger, significantly increasing the rate of fire.

 A bump stock installed on an AR-15 semi-automatic rifle. The device increases the rate of fire.

Videos posted on YouTube show users emptying entire clips in seconds.

“It’s an after-market kind of thing you can add to any rifle really, and get the same kind of fire out of a semi-automatic rifle,” said Russell Mahoney, owner of Topside Gun Warehouse in Blount County.

Mahoney said fully automatic weapons can cost anywhere from $20,000-60,000, and require dealers to carry additional certifications from the federal government. But a bump stock sells for several hundred dollars, and can be installed on the weapon easily. It allows a person to fire several rounds per second from a semi-automatic gun.

Russell Mahoney owns Topside Gun Warehouse in Blount County.

“In the two years we’ve been here, we’ve sold maybe three or four,” said Mahoney. He said the limiting factor for many people is that the rate you expend ammo can get expensive.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) heavily regulates who can buy and possess a machine gun. But many bump stocks are legal under ATF guidelines.

A spokesperson for the Tennessee Bureau of Investigation directed all questions to the ATF. ATF did not respond directly to requests for comment, but does review bump stock devices.

The bureau has previously denied some similar devices after finding them to be legally creating machine guns.

Mahoney said it’s difficult to shoot accurately using a bump stock.

NBC News reports it’s possible, but illegal, to permanently convert a semi-automatic rifle to full-automatic.

Some experts have also wondered if Paddock used a "trigger crank" to increase his rate of fire. They point to the speed of shots, and the inconsistent rate of fire. Trigger cranks sell online for around $50.

A trigger crank found online.

Mahoney said since neither of these devices are motorized, installing them does not change the semi-automatic nature of a weapon in the eyes of the ATF.

He noted many people use them safely for target shooting.

“Our position is, if it’s legal, it’s legal,” he said. “And if people operate them responsibly and safely, there’s no problem.”