They asked him, "What then? Are you Elijah?" He said, "I am not." "Are you the Prophet?" He answered, "No."
Art thou the prophet? ... is a reference to the prophet like unto Moses  (Deuteronomy 18:15-18) who must be identified with the Messiah.  This question therefore covers the same ground as the query, "Art thou the Christ?" which had already been answered.  It was the old reporter's trick of asking the same question again in different words, and John again answered it negatively.