Authorities said the black pistol in his backpack had a loaded Glock magazine with six nine-millimeter full metal jacket rounds. Police believe both the gun and the suppressor were 3D-printed. That revelation, coupled with Thompson’s brutal early-morning killing on a New York City street, have set off renewed debate about the ease of obtaining 3D-printed weapons that might ultimately wind up being used to commit crimes. Authorities announced on Wednesday that three shell casings found at the crime scene matched the gun Mangione was found with in Pennsylvania.

A report from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) revealed that the number of privately made firearms or “ghost guns” recovered by police in criminal cases rose more than 1,000%, from 2017 to 2021, ATF public affairs chief Kristina Mastropasqua told Fortune. During roughly the same period, more than 45,000 suspected ghost guns were reported to the ATF as having been recovered from possible crime scenes, including 692 homicides or attempted homicides, according to the bureau. In 2022 alone, more than 25,000 ghost guns were recovered. The ATF defines ghost guns or privately made firearms (PMFs) as firearms that have been built, assembled, or produced by a person other than a licensed gun manufacturer. 

“PMFs have been on the rise over the past decade,” Mastropasqua told Fortune. 

According to an ATF assessment, ghost guns in the 1980s and 1990s were made from stamped sheet metal that could be bent into shape using tools like hydraulic metal brakes, welding instruments and required technical expertise and experience, to boot. In the early 2000s, tech advances made it easier for people to make ghost guns from commercially available weapons kits. The ATF report revealed that Google and YouTube searches for ghost gun-related terms yielded more than 5 million pages of results and video views ranged between 1.3 and 8 million, depending on the length of time the video had been available. 

Ghost guns gained widespread public attention in 2013, when a California man killed six people on the Santa Monica College campus, including his father and brother. According to the Associated Press, John Zawahri cobbled together an AR-15-style weapon after he failed a background check at a licensed gun dealer. He was killed by police. 

A 2017 shooting involved a Northern California man who killed his wife and four other people after building his own gun. Two years later, a Los Angeles teenager shot and killed two classmates and wounded three more at a school. Just last week, a ghost gun critically wounded two kindergarteners at a small religious school in Northern California, according to law enforcement. 

Generally, it’s legal under federal law to make a gun with a 3D printer, as long as the person isn’t engaged in the business of making or dealing in guns, Mastropasqua wrote in a statement. If they are, they must have a federal firearms license. Before 2022, however, 3D gun-making kits could be sold without background checks or serial numbers, which made it incredibly challenging for police and law enforcement to trace the weapons used in crimes and identify and find shooters, she said. 

“As a result, PMFs are attractive to criminals and others legally prohibited from possessing firearms,” said Mastropasqua. 

In 2022, the ATF updated the regulatory definition of a firearm. The grip frame is the main part of a handgun and, in most designs, it holds the firing mechanism and is where the magazine is inserted. The receiver is the main body of a rifle or shotgun and holds the bolt mechanism. The 2022 ATF rule amendment updated the definition of a firearm frame and receiver to clarify that partially complete parts and kits that can be converted into functional firearms fall under the Gun Control Act. Therefore, companies selling so-called “buy, build, shoot” kits had to be licensed, add serial numbers, and run background checks on buyers. Guns accepted into inventory, including 3D printed weapons, now have to be marked with a serial number. 

However, Mastropasqua noted the rule is limited to the commercial sale of those guns and doesn’t apply to people who make ghost guns for their own use. Plus, the rule is subject to a significant legal challenge and remains an evolving area amid ongoing debate. In October, the Supreme Court heard oral arguments in a case challenging the ATF’s rule, although it remains in effect pending the decision, Mastropasqua said. 

In Thompson’s killing, authorities in Pennsylvania and New York have hit Mangione with weapons charges, in addition to a second-degree murder charge. Surveillance footage shows that Mangione crept up behind the 50-year-old father of two and fired into his back. 

“I have no tolerance, nor should anyone, for one man using an illegal ghost gun to murder someone because he thinks his opinion matters most,” Pennsylvania Gov. Josh Shapiro told the public on Monday.