Deuteronomy 21:9

In this way they were to deliver themselves as a nation from blood-guiltiness. "Expiation was made by the killing of the transgressor when he could be found ( Deuteronomy 19:13 ; Numbers 35:33 ); when he was not known, by the process here described. Of course, if afterwards he were apprehended, he would suffer the penalty he had incurred" (Knobel); so also Keil, Herxheimer, etc; after the Talmud ('Sota,' 9.7).