When authorities in Altoona, Pennsylvania, searched Luigi Mangione’s backpack Monday, they found a pistol with 3D-printed parts and a 3D-printed silencer, according to court records. Homemade firearms like the one allegedly found in Mangione’s backpack are a particular headache for local and federal law enforcement agencies.

They can be built from kits purchased online or with parts produced at home with a 3D printer. That means someone who may be otherwise unable to purchase or legally own a gun can still get one without showing ID or going through a background check, for example. And, unlike conventional firearms, they’re untraceable.

Known as “ghost guns,” homemade weapons lack serial numbers, which makes it almost impossible for authorities to determine their origin. When a gun store sells a firearm in the U.S., the serial number and purchaser are required to be recorded in a logbook under federal law.

When a gun is recovered at a crime scene or from a suspect, local authorities can ask the Bureau of Alcohol Tobacco, Firearms and Explosives, or ATF, to track down the gun’s provenance. 3D-printed firearms pose even more of a public safety issue, authorities have said, because of how cheap it is to mass produce them. The schematics for most of the needed parts are available for free online, and the printers themselves can cost less than $500. Filament needed to make many of the components is also relatively cheap. Because they are cheap they can also be of relatively low quality.

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Many of their components are made of plastic and the weapons are prone to jamming or misfiring. Some makes will fall apart after a few uses. When the gunman opened fire on UnitedHealthcare CEO Brian Thompson, security video shows the gun jammed. The jam was quickly cleared and the gunman kept firing. In just a few years, ghost guns have grown to become the most commonly recovered make of firearm in Baltimore. In 2022, President Joe Biden’s administration added regulations for ghost gun parts. The U.S. Supreme Court heard the legal challenge to those rules in October and has yet to issue a decision.