The ones who had been sent were from the Pharisees.
It was the baptizing and not the preaching which caused the greatest perplexity in John's questioners.  The extensive mass cleansing of the whole nation through repentance and baptism clearly suggested the great cleansing that had been prophesied by Ezekiel of the times of the Messiah  (Ezekiel 36:25; 37:23); why then was John doing it if indeed he was not Christ nor the kind of forerunner they expected to precede the Christ? This query shows that they had missed completely the implication of John's quoting  Isaiah 40:3, in which he made it clear that he was actually the forerunner of the Messiah, but not the literal Elijah they had expected.