What is a ghost gun and are they legal?

Shooter of Brian Thompson, the UnitedHealthcare CEO, was in possession of one made with a 3D printer when arrested

A person of interest has been identified in the fatal shooting of UnitedHealthcare CEO Brian Thompson. Police say they found a ghost gun" made with a 3D printer on the man, who has been charged in Pennsylvania with weapons, forgery and other crimes. The use of this type of firearm - whether made with a printer or bought online as a kit and assembled at home - has grown increasingly common in the past decade in part because they don't have serial numbers and can't be traced by authorities.

Here's how they've gone from a pastime of gun enthusiasts and tinkerers to an increasing part of US gun violence - and the subject of a major supreme court case.