What to know about ghost guns, the type of untraceable firearm CEO Brian Thompson's shooter may have used

Luigi Mangione, a person of interest being questioned in connection with the shooting death of UnitedHealthcare CEO Brian Thompson, had a ghost gun, a type of firearm that can be made at home, police said Monday.

Mangione was detained on Monday at a McDonald's in Altoona, Pennsylvania, early a week after Thompson was killed in New York City. Thompson was shot in the back and in the calf.

"As of right now the information we're getting from Altoona is that the gun appears to be a ghost gun, may have been made on a 3D printer, with the capability of firing a 9mm round," NYPD Chief of Detectives Joseph Kenny said Monday. "Obviously, that will come out during our ballistics testing."