The man was arrested on Monday In connection with the fatal shooting of UnitedHealthcare’s CEO, Brian Thompson was allegedly in possession of a type of homemade weapon known as a “ghost gun.”

26-year-old Luigi Mangione, whom police described as a “strong person of interest,” had a “ghost gun capable of firing 9mm cartridges and a silencer” when he was arrested on gun possession charges in Altoona, Pennsylvania York Police Chief Detective Joseph Kenny told reporters.

A senior police official told NBC News that the gun found in Mangione’s possession appeared to be similar to the gun used in the Manhattan murder. Kenny said the gun “may have been made using a 3D printer.”

What are Ghost Guns?

“Ghost guns” are firearms that can be assembled at home from parts purchased online. These parts are typically available without background checks and do not have serial numbers.

Since when have ghost guns existed?

The do-it-yourself kits have been around since the 1990s, but have become increasingly popular in recent years – especially among criminals.

The Federal Bureau of Alcohol, Tobacco, Firearms and Explosives said it received around 45,000 reports of suspected privately manufactured firearms that were seized by law enforcement during criminal investigations between January 2016 and December 2021 – including 692 homicides or attempted homicides.

According to the Justice Department, the number of ghost guns recovered rose steadily in each of those years, from 1,758 in 2016 to 19,344 in 2021.

Technology has also contributed to the sharp rise – online videos of how to assemble the guns are getting millions of views, while some sellers are offering 3D printing files that allow customers to print and assemble the guns themselves, without serial numbers.

The Justice Department said in a 2022 fact sheet that it is extremely difficult to track down non-digitized firearms – the ATF reported that they trace only 0.98% of suspected ghost guns submitted by law enforcement to a single buyer could.

What is the government doing about this matter?

In 2022, President Joe Biden announced restrictions on the sale of ghost guns and approved a rule requiring gun kit manufacturers to provide serial numbers on firearms and requiring sellers to adhere to the same standard as other gun purchases, including requiring background checks.

“These weapons are the weapons of choice for many criminals,” Biden said, and “we will do everything we can to take that choice away from them.”

Gun rights groups and manufacturers have challenged the ATF rule in court, with a federal judge in Texas and an appeals court ruling against the Biden administration. The Supreme Court has twice ruled that the rules can remain in effect while the litigation continues.

Meanwhile, gun safety advocates have called on Congress to take action on the issue to close loopholes and turn the rule into law. According to Everytown for a Gun Safety, a gun violence prevention organization, over a dozen states have also passed laws regulating ghost guns.

The group and others have also called on Congress to take action on 3D-printed guns, on which the Trump administration loosened regulations in 2020. There is no federal law banning them, but the ATF says it is illegal to offer them for sale without a license and that they can be detected by metal detectors and X-ray machines. According to Everytown, a number of states also have individual laws regulating or banning the weapons.