Mishnayos Sotah Perek 9 Mishnah 7

If the killer is found before the heifer’s neck was broken, the heifer shall go out and graze among the herd. It is not considered sacred at all, and it may rejoin the other animals. If the killer is found from the time when the heifer’s neck was broken, even if the rest of the ritual has not yet been performed, it is prohibited to benefit from the animal, despite the killer having been found; it should be buried in its place. This is because the heifer initially came for uncertainty, as the killer was unknown, and it atoned for its uncertainty and left, i.e., it fulfilled its purpose of bringing atonement and is considered a heifer whose neck is broken in all regards. If the heifer’s neck was broken and afterward the killer was found, he is killed. The ritual does not atone for him.

נִמְצָא הַהוֹרֵג עַד שֶׁלֹּא נֶעֶרְפָה הָעֶגְלָה, תֵּצֵא וְתִרְעֶה בָעֵדֶר. מִשֶּׁנֶּעֶרְפָה הָעֶגְלָה, תִּקָּבֵר בִּמְקוֹמָהּ, שֶׁעַל סָפֵק בָּאתָה מִתְּחִלָּתָהּ, כִּפְּרָה סְפֵקָהּ וְהָלְכָה לָהּ. נֶעֶרְפָה הָעֶגְלָה וְאַחַר כָּךְ נִמְצָא הַהוֹרֵג, הֲרֵי זֶה יֵהָרֵג: