The Supreme Court on Wednesday upheld a Biden administration regulation on the nearly impossible-to-trace weapons called ghost guns, clearing the way for continued serial numbers, background checks and age verification requirements for buying the kits online. The 7-2 opinion found that existing gun laws allow regulation of the kits increasingly linked to crime. It comes after President Donald Trump ordered a review of federal gun policy that could undermine or reverse regulations championed by his predecessor. Sales of the homemade firearms grew exponentially as kits allowing for easy at-home building came into the market, Justice Neil Gorsuch wrote in the majority opinion. “Some home hobbyists enjoy assembling them. But criminals also find them attractive,” he said. The number of ghost guns found at crime scenes around the country has also soared, according to federal data. Fewer than 1,700 were recovered by law enforcement in 2017, but that number grew to 27,000 in 2023, according to Justice Department data. Since the federal rule was finalized, though, ghost gun numbers have flattened out or declined in several major cities, including New York, Los Angeles, Philadelphia and Baltimore, according to court documents. Manufacturing of miscellaneous gun parts also dropped 36% overall, the Justice Department has said. Ghost guns are any privately made firearms without the serial numbers that allow police to trace weapons used in crime. The 2022 regulation was focused on kits sold online with everything needed to build a functioning firearm — sometimes in less than 30 minutes, according to court documents.