Police say UnitedHealthcare's CEO was likely killed with a ghost gun. What are they?

Luigi Mangione, the suspect charged in the murder of UnitedHealthcare CEO Brian Thompson, was carrying a "ghost gun" at the time of his arrest, authorities said.

The 26-year-old was "in possession of a ghost gun that had the capability of firing a 9 millimeter round" when he was arrested in Altoona, Penn., on Monday, New York Police Department (NYPD) Chief of Detectives Joseph Kenny said at a press briefing.

The NYPD said the gun, which is "consistent with the weapon used in the murder," may have been made on a 3D printer.

"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 said on Monday.

Ghost guns, also known as privately made firearms, are assembled by their owners, either from scratch or through weapon parts kits. They are not marked with serial numbers, making them easy for criminals to acquire and difficult, if not impossible, for law enforcement to trace.

The Department of Justice Bureau of Alcohol, Tobacco, Firearms and Explosives said in 2022 that within the previous five years, it was only able to successfully trace 0.98% of suspected "ghost guns" back to their individual purchaser.

Over the last decade, a growing number of ghost guns have been recovered from crime scenes across the U.S., worrying many authorities. They have been used in homicides, domestic violence, robberies, killings of law enforcement officers, mass shootings and school shootings, including one that wounded two kindergarteners at a Northern California religious school last week.

The advocacy organization Everytown for Gun Safety has called them "the fastest growing gun safety problem in the country."

While it is legal in the U.S. to build a firearm for personal use, the Biden administration, as well as more than a dozen states, have tried with varying degrees of success to regulate ghost guns.

Here's what to know.

How are ghost guns made?

There are several main methods for assembling a ghost gun, a process that gun control advocates say can take less than an hour and costs only a few hundred dollars.

One is to use a 3D printer — with the instruction manuals and videos easily available online — to create some or most of the parts from scratch.

People can also buy the necessary components online, either piece by piece or all together in what are called buy-build-shoot kits.

'Buy-build-shoot kits are weapon parts kits that are essentially pre-manufactured, [disassembled], complete firearms (a firearm in a box)," the U.S. Department of Justice says.

It was legal for retailers to sell those kits without running background checks until 2022, when the Justice Department passed a rule aimed at curbing the growing use of ghost guns in crimes.

How prevalent are ghost guns?

Ghost guns have been around in the U.S. at least since the 1990s, but have proliferated over the last decade or so.

The ATF says it received approximately 45,000 reports of suspected ghost guns recovered by law enforcement in criminal investigations between January 2016 and December 2021. Of those investigations, 692 involved homicides or attempted homicides.

Data from the bureau shows the number of suspected ghost guns rising steadily each year over that period, from 1,758 in 2016 to 19,344 in 2021.

State- and city-specific data also sheds light on the growing prevalence of ghost guns in recent years.

California data released in October shows that 8,340 ghost guns were recovered in the state in 2023, compared to just three in 2013.

Philadelphia police recovered 575 ghost guns in 2022, reporting a 311% increase in their use since 2019. The NYPD reported that officers seized 463 ghost guns in 2022, up from 263 the previous year.

"They are extremely dangerous and we must do more on the federal level to clamp down on the availability of ghost guns," New York City Mayor Eric Adams said at Monday's briefing.

How are ghost guns regulated?

Private U.S. citizens are allowed to build guns for personal use under the Gun Control Act of 1968.

Even so, some state and federal authorities are taking steps to crack down on ghost guns.

Fifteen states have passed laws to regulate them, with many requiring serial numbers and background checks for component parts, and others — including New York — going a step further by requiring ghost guns to be reported to authorities.

In 2022, a Justice Department rule took effect that made weapons parts kits subject to the same regulations as traditional firearms, including requiring commercial sellers to become federally licensed, mark certain parts with serial numbers and run background checks on purchasers.

The rule also aims to regulate some of the ghost guns already in circulation, by requiring federally licensed dealers and gunsmiths to put serial numbers on any guns they take into inventory that don't already have them, before selling them to another customer.

"If you commit a crime [with a] ghost gun, not only are state and local prosecutors going to come after you, but expect federal charges and federal prosecution as well," President Biden said that year.

Kit manufacturers and sellers challenged the rule in court, arguing the ATF exceeded its authority. The U.S. Supreme Court allowed the rule to remain in place pending litigation and heard the case in October.

It has not yet made a decision, though NPR's Nina Totenberg reported that the justices seemed inclined to side with the Biden administration.