Biblical Reference:
The concept of atonement for an unsolved murder is primarily found in the Old Testament, specifically in the book of Deuteronomy. The relevant passage is Deuteronomy 21:1-9.

Context and Procedure:
In ancient Israel, the shedding of innocent blood was a grave offense that defiled the land and required atonement to restore purity and justice. When a murder occurred and the perpetrator was unknown, the community was responsible for addressing the bloodguilt to prevent divine judgment.

The procedure for atonement is detailed in Deuteronomy 21:1-9. The passage outlines the steps to be taken when a slain person is found in the open country, and the murderer is not known:

1. Measurement to the Nearest City:
"If a man 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 killed him, your elders and judges must come out and measure the distance from the victim to the neighboring cities" (Deuteronomy 21:1-2). The elders and judges were to measure the distance to determine the nearest city to the body.

2. Involvement of the Elders and Priests:
The elders of the nearest city were to take responsibility for the atonement process. The Levitical priests, who were chosen by God to minister and pronounce blessings, were also involved, ensuring that the ritual was conducted according to divine law.

3. The Ritual of the Heifer:
"Then the elders of the city nearest the victim shall take a heifer that has never been yoked or used for work, bring the heifer down to a valley with running water, which has not been plowed or sown, and there in the valley they are to break the heifer’s neck" (Deuteronomy 21:3-4). The use of a heifer that had not been worked symbolized innocence and purity, and the breaking of its neck in an unworked valley signified the removal of guilt from the land.

4. Declaration of Innocence:
The elders were to wash their hands over the heifer and 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, O LORD, and do not hold Your people guilty of the blood of an innocent man" (Deuteronomy 21:7-8). This declaration was a public testimony of their innocence and a plea for divine forgiveness.

5. Divine Forgiveness and Cleansing:
The ritual concluded with the assurance that the bloodguilt would be atoned for, and the community would be absolved of the guilt of innocent blood. "So you shall purge from among you the guilt of shedding innocent blood, since you have done what is right in the eyes of the LORD" (Deuteronomy 21:9).

Theological Significance:
The atonement for an unsolved murder underscores the sanctity of human life and the seriousness with which God views the shedding of innocent blood. It reflects the communal responsibility to uphold justice and purity within the land. The ritual also foreshadows the ultimate atonement provided through Jesus Christ, whose sacrifice cleanses believers from all sin and guilt. The involvement of the priests and the symbolic acts point to the necessity of divine intervention and forgiveness in dealing with sin and its consequences.