The Supreme Court Fridayoverturned a federal agency’s order banning “bump stocks,” weapons used in some of America’s worst lone-wolf mass murders.

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

Plaintiff Michael Cargill, a gun shop owner from Austin, Texas, did not claim that the Second Amendment of the U.S. Constitution protected his right to own a bump stock. The case focused narrowly on the administrative procedure the ATF used to ban bump stocks, which use a firearm’s recoil to achieve rates of fire close to those of automatic weapons.

The ATF issued a regulation in 2018 that reclassified bump stocks as machine guns, making their possession illegal for civilians under federal law. The agency issued the regulation in response to the massacre at the Route 91 Harvest Music Festival in Las Vegas on October 1, 2017, when a lone gunman fired more than 1,000 shots into a concert crowd, killing 60 people and wounding 850 others.

However, the Supreme Court ruled that bump stocks did not meet the statutory definition of a “machine gun,” which requires that a gun must be fired automatically “by a single pull of the trigger.”

“A bump stock does not transform a semi-automatic rifle into a machine gun any more than a shooter with a lightning-fast trigger finger can transform it,” Judge Thomas wrote in an opinion peppered with illustrations documenting the workings of an AR-15 trigger.

Before the Las Vegas shooting, the ATF had ruled in at least ten individual cases that attaching a “bump stock” to a semi-automatic weapon would not turn it into a machine gun, the report states.

Thomas also called the ATF’s stance on bump stocks “logically inconsistent” because a shooter can fire a semi-automatic weapon using his hands alone, but the agency does not classify these bump-fire semi-automatic firearms as machine guns.

The court’s three liberal justices – Sonia Sotomayor, Elena Kagan and Katanji Brown Jackson – disagreed that bump stocks were similar enough to machine guns to warrant the ATF’s 2018 reclassification.

“This is not a difficult case,” Sotomayor wrote in a dissenting opinion. “All the textual evidence points to the same interpretation. A semiautomatic rifle equipped with a bump stock is a machine gun because (1) a shooter can fire continuous rounds with a single pull of the trigger (2) without requiring any human intervention other than maintaining forward pressure.”

The ruling in Garland v. Cargill is a major blow to gun reform advocates who viewed a ban on bump stocks as a common sense response to the lethality such devices can have in mass shootings.

“The majority of justices today sided with the gun lobby rather than standing up for the safety of the American people. This is a shameful decision,” wrote Esther Sanchez-Gomez, litigation director at the Giffords Law Center, in an emailed statement. “Congress must act to repair the harm and make clear that bump stocks and all automatic conversion devices are illegal under federal law.”

However, Friday’s ruling also made clear how fragile a ban imposed by the authority is. In many cases, reformers had feared that it would not withstand judicial review.

After the Las Vegas shooting, there was broad consensus that bump stocks, a small part of the overall gun industry, should be banned.

However, Congress did not immediately act to ban bump stocks after the Las Vegas shooting. Instead, then-President Donald Trump ordered the ATF to restrict the use of these devices.

The Supreme Court’s new ruling makes it clear that Congress would have to enforce a ban on bump stocks to remove the devices from the market.

“There is a simple remedy for the disparate treatment of bump stocks and machine guns,” Justice Samuel Alito wrote in a concurring opinion. “Congress can change the law — and might have done so already if the ATF had stuck to its earlier interpretation. Now that the situation is clear, Congress can act.”

Shortly after the ruling, Biden called on Congress to pass a ban, saying, “Americans should not have to live in fear of this mass devastation.”

“We know thoughts and prayers are not enough,” Biden’s statement said. “I call on Congress to ban bump stocks, pass an assault weapons ban, and take other actions to save lives – send me a bill and I will sign it immediately.”

It remains to be seen, however, whether such a measure can overcome Republicans’ historic resistance to gun reform in Congress, especially now that the sense of urgency following the Las Vegas shooting has faded.

Reporters asked Republican Rep. Byron Donalds (Florida), one of Trump’s biggest supporters in the House, whether there was interest in reinstating Trump’s ban on bump stocks.

“No, I don’t think so,” Donalds said. “I think it will probably stay where it is.”

Arthur Delaney contributed reporting.