Believe or not, the international gold standard for investigating potentially unlawful deaths where state involvement is suspected is called the "Minnesota Protocol". It's named like because it was developed by the Minnesota Lawyers International Human Rights Committee, in a very different and more human time (the 1990s). The protocol is built on the principle that the right to life is meaningless if there is no requirement for the state to investigate when that life is taken. It ensures that when someone is killed by police, military, or in state custody, there is a clear, objective path to uncovering the truth and holding the perpetrators accountable. It's basically a handbook for when and how to conduct such investigations. For an investigation to be credible, the protocol mandates four key pillars: it must be independent (no "police investigating themselves"), prompt (starting before evidence is lost), effective (a genuine search for the truth, not just going through the motions), and transparent (keeping the victim’s family and the public informed). Technically, the protocol is a comprehensive manual for forensics and law. It contains detailed guidelines on crime scene management, digital evidence collection, and forensic autopsies, such as tracking bullet trajectories to prove if a victim was shot while surrendering. It even includes specialized procedures for protecting witnesses from state retaliation.

You can learn more about it here (the latest version was published in 2016 and adopted by the UN).