Story highlights Bump stocks were approved for sale under a 2010 memo from the ATF

Bump stocks modify rifles like the AR-15 to allow it to fire in rapid succession

Washington (CNN) The Justice Department announced Tuesday that it has begun a federal rule-making process that could reinterpret the legality of certain bump fire stock devices, a piece of equipment that enabled the Las Vegas gunman in October to fire on concertgoers more rapidly, mimicking automatic fire.

The announcement comes the day before the acting director of the Bureau of Alcohol, Tobacco and Firearms is set to testify before a Senate Judiciary Committee hearing on the device and amid calls from legislators to prevent the future sale of the part.

Investigators have said 12 bump fire stock -- or bump stock -- devices were found in the shooter's hotel room in Las Vegas. Fifty-eight people died in the attack.

"Possessing firearm parts that are used exclusively in converting a weapon into a machine gun is illegal, except for certain limited circumstances," Attorney General Jeff Sessions said in a statement. "The regulatory clarification we begin today will help us to continue to protect the American people by carrying out the laws duly enacted by our representatives in Congress."

Bump stocks were approved for sale under a 2010 memo from the ATF's Firearms Technology Branch. Because the device "performs no automatic mechanical function when installed" and the shooter must still apply "constant" forward and rearward pressure to a gun equipped with one, the memo said the agency found the bump stock was a "firearm part," and therefore not regulated under federal gun laws.

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