The Prophet and the Messiah in John 1:20-21

Why did the Jews in John 1:20-21 ask John the Baptist if he was the Christ and then if he was the Prophet? Since the coming Prophet mentioned in Deut. 18:18 is the Messiah, it seems that their question was redundant. Or did they just not know that the Prophet and the Messiah were one in the same? I think I may just have answered my own question, but please elaborate.