The man suspected of the fatal shooting of UnitedHealthcare CEO Brian Thompson was carrying a “ghost gun” — a type of untraceable, homemade firearm — when he was apprehended Monday, police said.Related video above: What is a ghost gun?: ATF goes in depth on the rise of privately made guns and possible dangersLuigi Mangione had the gun when he was searched by local police at a Pennsylvania McDonald’s, New York Police Department Chief of Detectives Joseph Kenny said Monday, five days after the 26-year-old allegedly gunned down the executive outside a Midtown Manhattan hotel.The weapon was capable of firing a 9 mm round and “may have been made on a 3D printer,” Kenny said — a claim echoed by a criminal complaint filed in Pennsylvania, where police said they found a “black 3D-printed pistol” in Mangione’s backpack. Investigators also recovered a suppressor, which muffles the sound of gunfire, authorities have …