In 1957 Rodolfo Walsh published the first edition of Operación masacre, a thoroughly researched book that denounces the execution by the military régime of a group of civilians a year before. In the next few decades until his disappearance in 1977, Walsh would publish new expanded versions of the text that included documents that supported the veracity of his own investigation. Operación masacre is a rare case of where a political event engenders a new type of writing designed to tell the very story that has inspired it, and in so doing creates a new literary genre. In Operación Masacre, we read a text that is both a new writing and a new genre, produced in the urgency of political intervention, and with the desire to affect it and interpellate it. The relationship between literature and politics is dramatized in this text, as it will soon be dramatized in Walsh´s life. This chapter addresses the constitution of nonfiction in Walsh´s text, the relationships with his other works, and his legacy after his kidnapping and ensuing disappearance in 1977 at the hands of the military dictatorship.