Deuteronomy 21:7-8 -

This act they were to accompany with a solemn declaration of their innocence of this crime, and of their entire ignorance of the perpetrator of it; and with an earnest cry to God that the sin which had been done might be forgiven. Be merciful … unto ; be propitiated towards (literally, cover , כַּפֵּר לְעַמְךָ ; for the phrase, כַּפֵר לְ , see Le Deuteronomy 1:4 ). And lay not innocent blood ; the blood of the innocent man who has been slain.