WASHINGTON (AP) — The Supreme Court agreed on Monday to take up a Biden administration appeal over the regulation of difficult-to-trace ghost guns that had been struck down by lower courts.

The justices by a 5-4 vote had previously intervened to keep the regulation in effect during the legal fight. Ghost guns, which lack serial numbers, have been turning up at crime scenes with increasing regularity.

The regulation, which took effect in 2022, changed the definition of a firearm under federal law to include unfinished parts, like the frame of a handgun or the receiver of a long gun, so they can be tracked more easily. Those parts must be licensed and include serial numbers. Manufacturers must also run background checks before a sale, as they do with other commercially made firearms.

The requirement applies regardless of how the firearm was made, meaning it includes ghost guns made from individual parts or kits or by 3D printers. The rule does not prohibit people from buying a kit or any type of firearm.