Luigi Mangione might be a killer. He might not.

If police are right, however, he will be remembered as a herald of 3D-printed gun violence, of a weapon spreading so fast it is outpacing the ways we think about protecting ourselves.

Evidence is growing that the build-your-own weapon Pennsylvania police say they found on Mangione matches one used to kill the CEO of United Healthcare last week in New York. It is the highest-profile crime yet using this emerging technology.

People who work to curb gun violence and those who advocate for gun rights agree — controlling printed guns is beyond the reach of law enforcement and lawmakers.

“When we look at solutions for gun violence, we talk about policies to try to keep guns away from people who are experiencing high risks, at least temporarily,” said Alex McCourt of the Johns Hopkins Bloomberg School of Public Health in Baltimore. “3D guns skirt that.”

Where they disagree is how we should view the potential danger.

“With hysteria and a great rending of hair. I ask people to sit back in their chair,” laughed Mark Pennak, president of the gun rights group Maryland Shall Issue, “and ask them to think about 3D-printed guns.”

The 26-year-old Baltimore County man was arrested Monday in Altoona, Pennsylvania, and charged with the Dec. 4 sidewalk murder of Brian Thompson outside a Manhattan hotel. NYPD detectives said he had a ghost gun, capable of firing 9 mm rounds, that may have been made on a 3D printer.

3D guns are a subset of ghost guns, weapons manufactured without identifying serial numbers. Maryland outlawed ghost guns in 2022, and the U.S. Bureau of Alcohol, Firearms, Tobacco and Explosives required serial numbers on homemade guns the same year.

The U.S. Supreme Court heard a challenge to the federal rules in October, but the justices’ questions indicated that they would likely let them stand.

Both focus on the makers, not the ideas needed to make a 3D gun. That’s protected as free speech.

“I don’t know there’s a practical way to do that,” Pennak said.

It’s been 11 years since crypto-anarchist Cody Wilson designed the first 3D-printed gun. He made one on a printer purchased from eBay, test-fired it at a Texas range and then posted the open-source design, called G code, on his website. Within two days, 100,000 people had downloaded the plans.

The code and the printing technology have both advanced significantly since then.

“This is true of not just 3D guns but of ghost guns, or private-made firearms, in general,” McCourt said. “There are competitions on how well a 3D-printed gun can shoot on a range.”

Most “buy-build-shoot” guns come in a kit from a gun shop down the street or online with a barrel, bolt and recoil spring. The lower frame — the working chassis called a receiver — requires a serial number. You can buy those and assemble a working weapon in about an hour with common tools, moderate skills and some practice. Or you can print your own frame.

“There’s a hobbyist angle and personal freedom — look, I should be able to make my gun. I can build my own car, I can build my own gun,” McCourt said. “There’s a level of freedom inherent in that.”

That may be what Mangione did. Hobbyists online identified the gun linked to the shooting as a Chairmanwon V1, a 3D-printed frame combined with commercial parts that fire 9 mm rounds.

Pennak said there are still limits. Standard 9 mm rounds generate 35,000 psi, a force most polymers — the thermoplastic laid down in slices and fused through 3D printing — can’t withstand for long. Video of the shooting shows the gun may have jammed during the attack.

“The basic component that makes the gun go boom is really not going to be able to be fired without injuring the person who fires it,” Pennak said.

Yet, on Dec. 4, it worked. Fatally.

Go on any Reddit gun-making forum and you can see the ideas continuing to unfold. And the guns continue to spread.

Thompson’s death is not the first crime committed with a 3D-printed gun. Police recovered 45,240 homemade guns between 2016 and 2021, 692 of them from homicides or attempted homicides, according to a new review in Forensic Science International: Synergy. It’s a tiny, but growing fraction of the guns recovered yearly.

They’re a significant factor in Maryland. In 2022, Baltimore police recovered 402 guns made by Polymer80 — one of the nation’s largest kit gun makers — from crime scenes, a Baltimore Banner data review showed. In the first year of the state ban, police took in 373.

Polymer80 stopped selling or servicing its firearms in Maryland to settle a city lawsuit, but the guns didn’t disappear.

Statewide, police recovered 750 homemade guns this year through Dec. 10, a 3.3% increase from 2023, according to Maryland State Police data.

Some make guns for themselves, but others sell to people prohibited from owning them.

That might have been the case in St. Mary’s County last month. Sheriff’s deputies seized 80 guns and manufacturing equipment from a home in Lexington Park, many of them 3D-printed. The man charged with illegal firearms possession had a criminal record that barred him from owning a gun.

Others don’t want a record of their weapon for different reasons.

“I think there’s a sense of giving the finger to the man, which is out there,” said Pennak, who teaches classes on responsible gun ownership. “But when push comes to shove, people need to be able to defend themselves.”

The symbolism appeals to libertarian ideals and, beyond that, to revolutionary ones.

If Mangione killed Thompson, buying a gun would have been easier than building one. The choice feels like an anti-capitalist-techbro statement.

That fits with the words on bullet casings found where the shooter fired, “deny,” “delay” and “depose.” Mangione had a notebook of writings when police arrested him that seemed to confirm speculation that Thompson’s killing was motivated by anger at the insurance system.

Most home gun builders will never hurt anyone, and other weapons can be 3D printed, too.

A printed gun, though, used by a killer sending a message is something new. It seems likely to be repeated.

“Sure, guns are empowering. That’s why revolutionaries use them,” Pennak said. “But a single person with a gun is not going to be able to overthrow the government.”

All you have to do, then, is download the code and print more. That’s the danger.