At 2:10 in the afternoon of 25 July 1968, an explosion in a tank at the SIR (Societa Italiana Resine) petrochemical industry in Porto Torres (Sassari, Sardinia) caused the death of five people and the injury of five others. This article reports the events that led to the accident and explains the analysis activities that led to the reconstruction of the dynamics of the accident. The reconstruction of the etiology of the accident has been synthesized in an Incidental Sequence Diagram, a similar logical tree to the Fault Tree, in which the serious errors in the plant design are highlighted together with the poor management of some of the maintenance works.