Las Vegas mass shooting: What is a modified "bump stock" rifle?

CARY, North Carolina (WTVD) -- Investigators in Nevada on Tuesday confirmed the weapons owned by Las Vegas shooter 64-year-old Stephen Paddock included a modified "bump stock" rifle, which has an accessory that simulates automatic gunfire.

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At a Tuesday evening briefing, investigators said Paddock had devices attached to 12 weapons allowing semiautomatic rifles to mimic fully automatic gunfire.

Bureau of Alcohol, Tobacco and Firearms Special Agent in Charge Jill Schneider also told reporters Tuesday that Stephen Paddock had nearly 50 guns in three locations.

She said he had a combination of rifles, shotguns and pistols.

The gun attachment that mimics automatic gunfire is a little-known device called a "bump stock" that was not widely sold. The stocks have been around for less than a decade, and Schneider said officials determined they were legal.

The ABC11 I-Team checked with nearly a dozen gun stores and ranges in the Triangle and found none of them sell a bump stock accessory on location. Some places, like the Triangle Shooting Academy, don't even allow gun owners to use them at their facility.

Weighing less than one pound, the device instead can be found online, and sell for anywhere between $89 to $199. The simple technology of recoil allows an AR-15 rifle to fire at a faster rate than it could with a manual trigger pull.

One manufacturer, Slide Fire, posts a letter from the Bureau of Alcohol, Tobacco and Firearms explaining how and why the devices are legal:

"The stock has no automatically functioning mechanical parts or springs and performs no automatic function when installed... Accordingly, we find that the 'bump stock' is a firearm part and is not regulated as a firearm under Gun Control Act or the National Fireams Act."

In a video posted on its website, Slide Fire advertises the contraption's "power and functionality."

Las Vegas Police Department Assistant Sheriff Todd R. Fasulo said that 23 guns were found in the Mandalay Bay Hotel and Casino room where suspected shooter Stephen Paddock fired into a crowd, and 19 were discovered out of his Mesquite, Nevada home.

The Bureau of Alcohol, Tobacco, Firearms and Explosives released a statement announcing that it is "currently conducting an urgent trace on firearms recovered from the scene in Las Vegas."

The Associated Press contributed to this report.