Sforno on Deuteronomy 21:9/ספורנו על דברים כ״א:ט׳

1ואתה תבער, when you are aware of someone’s guilt it is up to you to sentence the party and to carry out the sentence and not to rely on heaven to intervene. When you are unaware of the identity of the guilty party, and you have made exhaustive attempts to find him without success, you may rest assured that G’d Himself will take care of the problem.