investigation should be opened into a complaint; the CPT welcomes such an approach. However, it is not unusual for the day-to-day responsibility for the operational conduct of an investigation to revert to serving law enforcement officials. The involvement of the prosecutor is then limited to instructing those officials to carry out inquiries, acknowledging receipt of the result, and deciding whether or not criminal charges should be brought. It is important to ensure that the officials concerned are not from the same service as those who are the subject of the investigation. Ideally, those entrusted with the operational conduct of the investigation should be completely independent from the agency implicated. Further, prosecutorial authorities must exercise close and effective supervision of the operational conduct of an investigation into possible ill-treatment by public officials. They should be provided with clear guidance as to the manner in which they are expected to supervise such investigations. 33. An investigation into possible ill-treatment by public officials must comply with the criterion of thoroughness. It must be capable of leading to a determination of whether force or other methods used were or were not justified under the circumstances, and to the identification and, if appropriate, the punishment of those concerned. This is not an obligation of result, but of means. It requires that all reasonable steps be taken to secure evidence concerning the incident, including, inter alia, to identify and interview the alleged victims, suspects and eyewitnesses (e.g. police officers on duty, other detainees), to seize instruments which may have been used in ill-treatment, and to gather forensic evidence. Where applicable, there should be an autopsy which provides a complete and accurate record of injury and an objective analysis of clinical findings, including the cause of death. The investigation must also be conducted in a comprehensive manner. The CPT has come across cases when, in spite of numerous alleged incidents and facts related to possible ill-treatment, the scope of the investigation was unduly circumscribed, significant episodes and surrounding circumstances indicative of ill-treatment being disregarded. 34. In this context, the CPT wishes to make clear that it has strong misgivings regarding the practice observed in many countries of law enforcement officials or prison officers wearing masks or balaclavas when performing arrests, carrying out interrogations, or dealing with prison disturbances; this will clearly hamper the identification of potential suspects if and when allegations of ill-treatment arise. This practice should be strictly controlled and only used Appendices ► Page 77