The Supreme Court on Monday ordered two internet sellers of gun parts to comply with a Biden administration regulation aimed at "ghost guns," firearms that are difficult to trace because they lack serial numbers.

The court had intervened once before, by a 5-4 vote in August, to keep the regulation in effect after it had been invalidated by a lower court. In that order, Chief Justice John Roberts and Justice Amy Coney Barrett joined with the three liberal justices to freeze the lower court's ruling. Justices Clarence Thomas, Samuel Alito, Neil Gorsuch and Brett Kavanaugh said they would deny the request from the Biden administration to revive the rules.

No justice dissented publicly from Monday's brief, unsigned order, which followed a ruling from a federal judge in Texas that exempted the two companies, Blackhawk Manufacturing Group and Defense Distributed, from having to abide by the regulation of ghost gun kits.

Other makers of gun parts also had been seeking similar court orders, the administration told the Supreme Court in a filing.

"Absent relief from this Court, therefore, untraceable ghost guns will remain widely available to anyone with a computer and a credit card — no background check required," Solicitor General Elizabeth Prelogar, the administration's top Supreme Court lawyer, wrote.

The regulation 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 regulation will be in effect while the administration appeals the judge's ruling to the 5th U.S. Circuit Court of Appeals in New Orleans — and potentially the Supreme Court.

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