Luigi Mangione, the suspect charged with 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 capable of firing a 9-millimeter cartridge” when he was arrested by New York Police Department (NYPD) Chief Detective Joseph Kenny on Monday in Altoona, Pennsylvania was said at a press conference.

The NYPD said the gun, which “matches the weapon used in the murder,” may have been made using a 3D printer.

“I have no tolerance, nor should anyone, for a man to use an illegal ghost gun to murder someone because he believes his opinion is most important,” Pennsylvania Gov. Josh Shapiro said Monday.

Ghost guns, also called privately manufactured firearms, are assembled by their owners either from scratch or through gun parts kits. They are not marked with serial numbers, making them easy for criminals to obtain and difficult, if not impossible, for law enforcement to track down.

The U.S. Department of Justice’s Bureau of Alcohol, Tobacco, Firearms and Explosives said in 2022 that it had successfully traced only 0.98% of suspected “ghost guns” to their individual purchaser over the past five years.

Over the past decade, an increasing number of ghost guns have been recovered from crime scenes in the United States, worrying many authorities. They have been used in homicides, domestic violence, robberies, police killings, mass shootings and school shootings, including a shooting that injured two kindergarten teachers at a religious school in Northern California last week.

The advocacy group Everytown for Gun Safety has called it “the fastest-growing gun safety issue in the country.”

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

Here’s what you should know.

How are ghost weapons made?

There are several main methods for assembling a ghost weapon. Gun control advocates say this process can take less than an hour and costs just a few hundred dollars.

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

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

“Buy-build shoot kits are gun parts kits that are essentially pre-built (disassembled) complete firearms (a firearm in a box),” according to the U.S. Department of Justice.

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

How common are ghost weapons?

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

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

Bureau data shows the number of suspected ghost guns rising steadily each year during this period, from 1,758 in 2016 to 19,344 in 2021.

State- and city-specific data also shed light on the increasing 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 seized 575 ghost guns in 2022, 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 at the federal level to curb the availability of ghost guns,” New York Mayor Eric Adams said at Monday’s press conference.

How are ghost weapons regulated?

Under the Gun Control Act of 1968, private U.S. citizens are permitted to build guns for personal use.

Still, some state and federal agencies are taking action to crack down on ghost guns.

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

In 2022, a Department of Justice rule went into effect requiring gun parts kits to be subject to the same regulations as conventional firearms. This includes requiring commercial sellers to obtain a federal license, mark certain parts with serial numbers and conduct background checks on buyers.

The rule also aims to regulate some of the ghost guns already in circulation by requiring federally licensed dealers and gunsmiths to affix serial numbers to any guns they stock that are not already in existence before selling them sell to another customer.

“If you commit a crime (with a ghost gun), not only will state and local prosecutors go after you, but you will also face federal charges and prosecution,” President Biden said that year.

Kit manufacturers and sellers challenged the rule in court, arguing that the ATF overstepped its authority. The U.S. Supreme Court allowed the rule to remain in effect pending trial and heard the case in October.

No decision has been made yet, although NPR’s Nina Totenberg reported that the justices appear inclined to side with the Biden administration.