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).