Elijah is come already (Eleias ēdē ēlthen). Thus Jesus identifies John the Baptist with the promise in Malachi, though not the real Elijah in person which John denied (Joh 1:21).
They knew him not (ouk epignōsan auton). Second aorist active indicative of epiginōskō, to recognize. Just as they do not know Jesus now (Joh 1:26). They killed John as they will Jesus the Son of Man.