NEW: U.S. Attorney James McDonald speaks moments after Luigi Mangione pleads guilty and admits to killing UnitedHealthcare CEO Brian Thompson, laying out what p

NEW: U.S. Attorney James McDonald speaks moments after Luigi Mangione pleads guilty and admits to killing UnitedHealthcare CEO Brian Thompson, laying out what prosecutors describe as careful preparation leading up to the murder. "This was not a spontaneous act of violence. It was deliberate, calcu…