Deuteronomy 21:1-9

If someone is found slain, lying in a field in the land the LORD your God is giving you to possess, and it is not known who the killer was, your elders and judges shall go out and measure the distance from the body to the neighboring towns. Then the elders of the town nearest the body shall take a heifer that has never been worked and has never worn a yoke and lead it down to a valley that has not been plowed or planted and where there is a flowing stream. There in the valley they are to break the heifer’s neck. The Levitical priests shall step forward, for the LORD your God has chosen them to minister and to pronounce blessings in the name of the LORD and to decide all cases of dispute and assault. Then all the elders of the town nearest the body shall wash their hands over the heifer whose neck was broken in the valley, and they shall declare: “Our hands did not shed this blood, nor did our eyes see it done. Accept this atonement for your people Israel, whom you have redeemed, LORD, and do not hold your people guilty of the blood of an innocent person.” Then the bloodshed will be atoned for, and you will have purged from yourselves the guilt of shedding innocent blood, since you have done what is right in the eyes of the LORD.