Four investigating officers and one crime analyst from two police departments and a university public safety department were interviewed for this study in the summer and fall of 1984. The contents of case and service reports dealing with 19 sexual offenses were discussed in these interviews. The investigator's normal routine is guided by the formal training and manual rules for investigation along with the officer's own cognitive structures, which facilitate the identification of normal patterns and the plotting of strategies for collecting evidence. Regarding cognitive structures and interpretive procedures in police investigations, effective investigation presumes a high level of 'interactional competence.' According to Fuller and Jacobs, interactional competence is based on three cognitive structures: reflexivity, which refers to the mind's ability to recall a past, be aware of the present, and project a hypothetical future; typification schemes, which refer to the ability to form categories for the classification, recognition, and understanding of experience; and interpretive procedures, which describe properties of the mind which enable humans to construct and interpret social interaction. This paper discusses how these cognitive structures were used by the interviewees in their investigations of sexual assaults. 35 notes and 19 references.