Washington  -  Gun rights activist Rob Pincus was worried when he heard a 3D-printed “ghost gun” was found on the alleged shooter in last week’s notorious murder of a US health executive in New York. Not because it showed the danger of untraceable weapons -- but because the killing of UnitedHealthcare CEO Brian Thompson would mean “another wave of attacks” against private gun makers. A police report states that 26-year-old Luigi Mangione, who led authorities on a six-day manhunt only to be arrested in a McDonald’s restaurant in Pennsylvania, was found with a “black 3D-printed pistol and a black silencer” in his backpack. The news sparked renewed fears over ghost guns, which President Joe Biden’s administration has tried to crack down on as the homemade weapons are increasingly used in crimes. Ghost guns are weapons put together as a kit or from separate pieces, sometimes made by 3D printers, and have no serial numbers so cannot be tracked. Anti-gun violence group Giffords Law Center slammed them as a “criminal’s dream come true,” while another group, Everytown, said the New York killing showed how they have “exacerbated our nation’s gun violence epidemic.”