The analysis proposed in this article aims at showing how it happened that the disappearance of forty-three students of École Normale rurale d’Ayotzinapa became « une affaire ». We examine how local claims could thus emerge on a larger scale and become publicly known issues, linked to social mobilization in Mexico and elsewhere. The two main sources used in order to gain an understanding of this event are police or court records and testimonies from the victims, as reported by the media in the days following the disappearance.