Regarding the "code":

I'm pretty confident that the FBI is not the inventor of it, so obviously there should be some information out there about "encoding a message to that the encoded message is human language".

Obviously there's a mapping (assuming x,y are unknowns)

(x | "through good ole Unk") -> "by skyjacking a jet plane".

(y | "And please tell the lackey cops") -> "I am 1st Lt. Robert Rackstraw,"

Doesn anyone know candidates for this type of mapping?