The Supreme Court decided on Monday to hear an appeal by the Biden administration regarding the regulation of ghost guns, which are difficult to trace and had been ruled down by lower courts.

The Justices had intervened in the past, by a vote of 5-4, to keep the regulations in place during the legal battle. Ghost guns, without serial numbers, are increasingly common at crime scenes.

This regulation came into effect in 2022 and changed the definition of firearms under federal law. It now includes unfinished parts like the handgun frame or receiver, to make it easier to track them. These parts must have serial numbers and be licensed. The manufacturers must conduct background checks on all firearms before they are sold, just as they do for other commercially produced guns.

This requirement is applicable regardless of the method by which a firearm was manufactured, including ghost guns that are made using individual parts, kits or 3D printers. The rule does allow people to purchase a firearm kit or other types of firearms.

The Justice Department told the court local law enforcement agencies had seized over 19,000 ghost weapons at crime scenes by 2021. This was a tenfold rise in only five years.

U.S. district judge Reed O’Connor, of Fort Worth, Texas, ruled against the rule in the past year, concluding it was beyond the Bureau of Alcohol, Tobacco, Firearms and Explosives’s authority. O’Connor wrote in his ruling that federal law’s definition of firearm does not include all parts of the gun. He wrote that Congress could change laws.

The 5th U.S. Circuit Court of Appeals, which is composed of three appointees of then-President Donald Trump, largely upheld O’Connor’s ruling. Three appointees of Donald Trump, then president of the United States, made up the 5th Circuit Court of Appeals largely upheld O’Connor’s decision.

The Supreme Court permitted the regulation to continue in effect as long as the lawsuit continued. The majority of the Supreme Court was formed by Chief Justice John Roberts, Justice Amy Coney Barrett and the three liberals. The four liberal justices, Brett Kavanaugh, Neil Gorsuch and Clarence Thomas, would have put the regulation on hold while the appeals process was underway.

Trump nominated Barrett, Gorsuch, and Kavanaugh.

Arguments will not take place until after the fall.