Elijah is come already (Ελειας ηδη ηλθεν). Thus Jesus identifies John the Baptist with the promise in Malachi, though not the real Elijah in person which John denied (John 1:21).

They knew him not (ουκ επιγνωσαν αυτον). Second aorist active indicative of επιγινωσκω, to recognize. Just as they do not know Jesus now (John 1:26). They killed John as they will Jesus the Son of Man.