This article explores different realizations of modality meanings in Spanish written language and their contribution to dialogic positionings in the field of the historical explanations provided in The National Commission for Truth and Reconciliation Report (1991) regarding human rights violations during Pinochet's dictatorship in Chile (1973-1990). From an ideological, socio-semiotic perspective anchored in the theoretical framework provided by Systemic Functional Linguistics, the analysis focuses on congruent and metaphorical expressions of modality in the text and their contribution to authors' positionings as organized in the system of engagement, considering, at the lexicogrammatical level, the place of modality in the interpersonal, ideational, and logical structure of the clause. The analysis shows the productive role of a range of realizations of modality meanings in the discourse to construe an interpretation of a conflicting, traumatic recent past and to present and validate the authorial voice regarding its mission and actions. An outline of grammatical interpersonal metaphors of modality for Spanish written language is also presented.