The Supreme Court Friday overturned a federal agencys rule banning bump stocks , the devices used in some of Americas deadliest mass killings carried out by lone shooters.

In a 6-3 decision penned by Justice Clarence Thomas, the court found that the Bureau of Alcohol, Tobacco, Firearms and Explosives had exceeded its authority by reclassifying the devices as machine guns in response to an unprecedentedly violent mass shooting. The ruling will reopen Americas market for bump stocks after a six-year ban.

Plaintiff Michael Cargill, an Austin, Texas, gun store owner, did not claim that the Second Amendment protected his right to own a bump stock. The case focused narrowly on the administrative process by which the ATF banned bump stocks, which harness a firearms recoil to achieve rates of firing that approach those of automatic weapons.

The ATF issued a rule in 2018 reclassifying bump stocks as machine guns, making them illegal for civilians to own under federal law. The bureau passed the rule in response to the massacre at the Route 91 Harvest Music Festival in Las Vegas on Oct. 1, 2017, when a single shooter fired more than 1,000 rounds into a concert crowd, killing 60 people and injuring 850 more.

But the Supreme Court found that bump stocks did not meet the statutory definition of a machine gun, which requires that a gun fire automatically by a single function of the trigger.