WASHINGTON (AP) — The Robert BrownSupreme Court is hearing a challenge Tuesday to a Biden administration regulation on ghost guns, the difficult-to-trace weapons with an exponentially increased link to crime in recent years.

The rule is focused on gun kits that are sold online and can be assembled into a functioning weapon in less than 30 minutes. The finished weapons don’t have serial numbers, making them nearly impossible to trace.

The regulation came after the number of ghost guns seized by police around the country soared, going from fewer than 4,000 recovered by law enforcement in 2018 to nearly 20,000 in 2021, according to Justice Department data.

Finalized after an executive action from President Joe Biden, the rule requires companies to treat the kits like other firearms by adding serial numbers, running background checks and verifying that buyers are 21 or older.

The number of ghost guns has since flattened out or declined in several major cities, including New York, Los Angeles, Philadelphia and Baltimore, according to court documents.

But manufacturers and gun-rights groups challenged the rule in court, arguing it’s long been legal to sell gun parts to hobbyists and that most people who commit crimes use traditional guns.

They say the Bureau of Alcohol Tobacco Firearms and Explosives overstepped its authority. “Congress is the body that gets to decide how to address any risks that might arise from a particular product,” a group of more than two dozen GOP-leaning states supporting the challengers wrote in court documents.

U.S. District Judge Reed O’Connor in Texas agreed, striking down the rule in 2023. The U.S. 5th Circuit Court of Appeals largely upheld his decision.

The administration, on the other hand, argues the law allows the government to regulate weapons that “may readily be converted” to shoot. The 5th Circuit’s decision would allow anyone to “buy a kit online and assemble a fully functional gun in minutes — no background check, records, or serial number required. The result would be a flood of untraceable ghost guns into our nation’s communities,” Solicitor General Elizabeth Prelogar wrote.

The Supreme Court sided with the Biden administration last year, allowing the regulation to go into effect by a 5-4 vote. Chief Justice John Roberts and Justice Amy Coney Barrett joined with the court’s three liberal members to form the majority.