Sorry if anybody has already said this, but I just thought of this.??If you remember in the episode the Drill the FN tried to use a drill to break few the walls of Ba-Sing-Se.??I started to think who built the drill today. I came up with a theory that the FN captured the Northern Air Temple after the GAang left. The FN then took the mechanist captive and forced him to make The Drill.
It is just a theory but it seems cool. And I am sorry if anybody already suggestted this.