Police investigating the shooting of UnitedHealthcare CEO Brian Thompson on Dec. 4, have announced that the suspected assailant had used a 3D-printed gun . Several high-profile crimes in recent years have involved this kind of homemade, or partially homemade, weapon. Often called " ghost guns " because they can be hard to trace, these firearms can be either partially or completely made with components that have been produced in metal or plastic on commercially available 3D printers. The U.S.