The suspect charged in the murder of UnitedHealthcare CEO Brian Thompson was carrying a so-called ghost gun when he was arrested in Pennsylvania this week. The firearms are legal to make in the U.S., with some caveats, raising questions about their potential for criminal use.

Luigi Mangione, the shooting suspect, was carrying a “ghost gun” that had the capability of firing a 9 millimeter round and may have been made on a 3-D printer, according to investigators.

The term ghost gun is used because they don’t have serial numbers, unlike those manufactured by licensed companies. Guns made by licensed businesses must have serial numbers, according to federal law, that allow them to be traced back to the maker, dealer and purchaser.

But if you want to just make your own gun at home, it is legal and relatively easy to do so.

A quick internet search, credit card payment, and everything needed to make a firearm can be delivered to the front door.

The homemade weapons are deadly like any store-bought firearm, and they have sometimes found their way into the wrong hands. That has prompted more regulation in recent years as the so-called ghost guns have been used in various crimes across the country, including in metro Atlanta.

Until about two years ago, ghost gun kits available online didn’t require background checks or age verification. The Biden administration added age requirements and background checks in 2022.

The law makes it harder for those with felony records, who are forbidden from having firearms, to make their own ghost guns. Still, crimes committed with homemade weapons are continuing, law enforcement groups say.

Everytown for Gun Safety, an advocacy group, has called ghost guns “the fastest growing gun safety problem in the country.”

“Buy-build-shoot” kits are essentially pre-manufactured, dissembled, complete firearms,” according to the U.S. Department of Justice.

A spokesman for the ATF previously told The Atlanta Journal-Constitution the agency doesn’t use the term “ghost guns” but instead calls them privately manufactured weapons, or PMFs. The agency says those wanting to make their own weapons for themselves can do so legally, and technology such as 3-D printing makes it even easier now. If a gun is made with the intention of selling it, though, that’s illegal.

“PMFs are commonly referred to as ‘ghost guns’ because it can be difficult to track them,” the ATF’s website states. “Investigating crimes involving unserialized PMFs can create difficulty in tracing the origins of the firearm and linking them to related crimes.”

From January 2016 to December 2021, the ATF received approximately 45,000 reports of suspected ghost guns or PMFs recovered by law enforcement in criminal investigations, the agency reported. Of those, 692 cases were homicides or attempted homicides.

In metro Atlanta, law enforcement agencies have also investigated cases involving ghost guns.

In 2021, a 13-year-old boy was arrested in Douglas County after shooting and killing his older sister, The AJC previously reported. The boy, later identified as Wilson Scott, was making guns and selling them, the sheriff’s office said at the time.

When a potential buyer tried to take a gun without paying for it, the teenager fired a shot with one of his homemade weapons. The shot killed 14-year-old Kyra Scott.

Both Wilson Scott and the robbery suspect, Yusef McArthur El, were both arrested after the girl’s death. Both later received probation, court records showed.

“A 13-year-old kid, doesn’t weigh but about 80 pounds, was able to make a weapon from start to finish,” Douglas County Sheriff Tim Pounds said following the arrests. “At 13 years old.”