4. Investigative mechanisms 38. The duty to investigate does not necessarily call for one particular investigative mechanism in preference to another. States may use a wide range of mechanisms consistent with domestic law and practice, provided those mechanisms meet the international law requirements of the duty to investigate. Whether a police investigation, a coronial inquest, an investigation by an independent police oversight body, an investigation by a judge, special prosecutor or national human rights institution, or any other investigation, complies with the duty to investigate, is a matter to be determined in the light of the international legal obligations and commitments of the State. Whichever mechanisms are used, however, they must, as a whole, meet the minimum requirements set out in these Guidelines. 40. States must ensure that special mechanisms do not undermine accountability by, for example, unduly delaying or avoiding criminal prosecutions. The effective conduct of a special investigative mechanism – designed, for example, to investigate the systemic causes of rights violations or to secure historical memory – does not in itself satisfy a state’s obligation to prosecute and punish, through judicial processes, those responsible for an unlawful death. Accordingly, while special mechanisms may play a valuable role in conducting investigations in certain circumstances, they are unlikely on their own to fulfil the State’s duty to investigate. Fulfilment of that duty may require a combination of mechanisms. 39. In specific circumstances a State may establish a special mechanism such as a commission of inquiry or another transitional justice mechanism. An international investigation mechanism with the expertise and capacity to conduct an independent and objective investigation may be appropriate. The requirements of promptness, effectiveness and thoroughness, independence and impartiality, and transparency apply equally to investigations undertaken by these mechanisms.83 83 10 In designing such mechanisms, States should have regard to the principles relating to commissions of inquiry contained in the Set of Principles to Combat Impunity, the OHCHR’s Commissions of Inquiry and Fact-Finding Missions Guidance and Practice and the Siracusa Guidelines for International, Regional and National Fact-Finding Bodies. The Minnesota Protocol on the Investigation of Potentially Unlawful Death (2016)