John 1:21

They asked him, "Then who are you? Are you Elijah?" He said, "I am not." "Are you the Prophet?" He answered, "No."

Malachi 4:5 See I will send you the prophet Elijah before the dreadful day of the Lord comes.

Deuteronomy 18:18 I will raise up for them a prophet like you from among their brothers; I will put my words in his mouth, and he will tell them everything I command him.

The scribes were harking back to these Old Testament prophesies. We know that John was fulfilling these prophesies, but not in the literal way that the Jews were expecting. Luke 1:17 tells us that John would be in the spirit of Elijah and he was called Elijah by Jesus himself (Matt 17:12). There have been many prophets who have fulfilled the Deuteronomy text, not least Elijah himself, but also Isaiah, Jeremiah, Ezekiel and many more. John the Baptist is but one and the mythology of the prophet was a construction of those who spend too much time with their heads in books.