Suspect in UnitedHealthcare CEO killing allegedly boasted making ghost gun: Sources

In a three-page handwritten note seized during his arrest, Luigi Mangione, the suspect in the targeted New York City shooting of UnitedHealthcare CEO Brian Thompson, appeared to boast about the do-it-yourself 3D-printed ghost gun he allegedly used in the brazen homicide, law enforcement sources told ABC News.

The 26-year-old Mangione, who studied computer science and earned bachelor's and master's degrees in engineering from the University of Pennsylvania, is alleged to have written in the document how easy it was to print out some of the components used to assemble the mostly plastic gun.

"This was fairly trivial: some elementary social engineering, basic CAD [Computer-aided Design], and a lot of patience," reads the document seized by police when Mangione was caught Monday at a McDonald's in Altoona, Pennsylvania, law enforcement sources told ABC News.

Apparently referring to the gun used in the high-profile homicide, the document found on Mangione added, "My tech is pretty locked down because I work in engineering."

A criminal complaint filed in court by Altoona police alleges officers who arrested Mangione following a five-day manhunt found a black 3D-printed pistol and a black silencer that was also 3D-printed in a backpack he had with him.

"The pistol had a metal slide and a plastic handle with a metal threaded barrel. The pistol had one loaded Glock magazine with six nine-millimeter full metal jacket rounds," according to the criminal complaint.

New York Police Commissioner Jessica Tisch said Wednesday that the three shell casings recovered at the scene of Thompson's shooting matched the gun found in Mangione possession when he was arrested. She also confirmed that Mangione's fingerprints were recovered from a water bottle and the wrapper of a Kind bar found near the crime scene.

Ghost guns, or privately made firearms without serial numbers, have surged in popularity in recent years. Due to the difficulty -- or even impossibility -- of tracing their origins, they can be an ideal weapon for those not legally permitted to own a gun, such as convicted felons or minors. Because of this, they are frequently the weapon of choice used in crimes.

Many ghost guns are homemade, typically 3D printed or assembled from kits that can be easily purchased online.

Due to their lack of serial numbers, it is impossible to know exactly how many exist, but thousands have been recovered from crime scenes in recent years.

About 45,240 suspected ghost guns were recovered from crime scenes between 2016 and 2021 -- 692 of which were homicides or attempted homicides, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives.

In 2022, the Department of Justice said it had "recovered 25,785 ghost guns in domestic seizures, as well as 2,453 through international operations."

Marcus Sachs, senior vice president and chief engineer for the Center for Internet Security headquartered in East Greenbush, New York, told ABC News that significant advances have been made in 3D printing in the past decade.

"Printing guns is just one example. Just about any weapon now can be printed," Sachs said.

Sachs said materials other than plastic, including ceramics and powered metals, can be used to print objects.

He said Auburn University has a 3D printer big enough to print automobiles.

"They're experimenting with that one, looking at how can we put one on the moon or on Mars and then use local material off the moon or Mars to print parts," Sachs said. "Rather than shipping a spaceship full of parts, you just ship a 3D printer and if you need a sprocket, you just melt some lunar dust and build your sprocket."

Sachs said someone like Magione, who studied mechanical engineering and computer science, would have likely been exposed to 3D printing.

"In most colleges now, that's either a required course or an elective in advanced manufacturing as they like to call it, but it's essentially 3D printing," Sachs said. "Being mechanical -- firearms are mechanical -- understanding basic laws of physics, properties and materials, it would not be difficult for an engineering student to understand those concepts and figure out how to build something like this."

Though ghost guns are legal in the U.S., in recent years, lawmakers have begun pushing for stronger legislation to curb the proliferation of these firearms.

In 2022, President Joe Biden announced new measures that would require gun kits to include serial numbers -- a move that has been swept up in litigation ever since as manufacturers fight regulation.

In October, the U.S. Supreme Court heard oral arguments in a case called Garland v. VanDerStok stemming a decision by the Fifth Circuit Court of Appeals to strike down portions of the Biden administration's 2022 measures to regulate ghost guns.

According to gun control advocacy group Everytown -- which has called ghost guns the "fastest-growing gun safety problem facing our country" -- there are 15 states that currently have laws governing the use of these firearms.

Many of these states require ghost guns to have serial numbers, and for their owners to go through background checks. Some require owners to disclose their ghost guns to officials.

But Sachs said most criminals aren't going to follow such laws and that catching people printing 3D guns is difficult.

"Rather than shipping weapons in a box that could be discovered by local authorities or customs, I can just email you instructions and you can make it right there, completely evading any sort of detection," Sachs said.

Sachs added, "For us in the security world, this is kind of a hybrid threat. You've got the intersection of computer science, the internet and computer security intersecting with the physical world and physical security."