A.T. Robertson Commentary Matthew 17:12

"but I say into you, that Elijah is come already, and they knew him not, but did unto him whatsoever they would. Even so shall the Son of man also suffer of them." — Matthew 17:12 (ASV)

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.