From the Principles on the Effective Prevention and Investigation of Extra-Judicial, Arbitrary, and Summary Executions, recommended by the United Nation Economic and Social Council Resolution 1989/65 of 24 May 1989.
Principle 9 outlines the manner in which such an investigation should be conducted:
There shall be thorough, prompt and impartial investigation of all suspected cases of extra-legal, arbitrary and summary executions, including cases where complaints by relatives or other reliable reports suggest unnatural death in the above circumstances. Governments shall maintain investigative offices and procedures to undertake such inquiries. The purpose of the investigation shall be to determine the cause, manner and time of death, the person responsible, and any pattern or practice which may have brought about that death. It shall include an adequate autopsy, collection and analysis of all physical and documentary evidence and statements from witnesses. The investigation shall distinguish between natural death, accidental death, suicide and homicide.
Principle 11 states that
In cases in which the established investigative procedures are inadequate because of lack of expertise or impartiality, because of the importance of the matter or because of the apparent existence of a pattern of abuse, and in cases where there are complaints from the family of the victim about these inadequacies or other substantial reasons, Governments shall pursue investigations through an independent commission of inquiry or similar procedure.
Principle 19 states that:
Without prejudice to principle 3 above, an order from a superior officer or a public authority may not be invoked as a justification for extra-legal, arbitrary or summary executions. Superiors, officers or other public officials may be held responsible for acts committed by officials under their authority if they had a reasonable opportunity to prevent such acts. In no circumstances, including a state of war, siege or other public emergency, shall blanket immunity from prosecution be granted to any person allegedly involved in extra-legal, arbitrary or summary executions.40
Principle 8 provides for an intergovernmental and/or international dimension to such investigations and to efforts
to prevent extra-judicial, arbitrary and summary executions through measures such as diplomatic intercession, improved access of complainants to intergovernmental and judicial bodies, and public denunciation. Intergovernmental mechanisms shall be used to investigate reports of any such executions and to take effective action against such practices.
 Principle 3 states that: Governments shall prohibit orders from superior officers or public authorities authorizing or inciting other persons to carry out any such extra-legal, arbitrary or summary executions. All persons shall have the right and the duty to defy such orders. Training of law enforcement officials shall emphasize the above provisions.