Numbers 35:15

These six cities shall be a refuge, both for the children of Israel and for the stranger and for the sojourner among them, that every one that killeth any person unawares, unintentionally, may flee thither. These regulations are still further expanded in Deuteronomy 19:1, especially with regard to the avenger of blood, for so the nearest relative of any person to whom an injustice had been done was called. By God's permission such a person was empowered to revenge a wrong, even to the extent of putting a slayer to death, the present precept regulating the custom. There follow regulations concerning the legal process in the case of manslaughter.