On 2 March 2015, one span of the so-called “Italia Viaduct”, the second highest bridge in Europe, collapsed during some renovation works, killing one worker. This article describes in detail the forensic investigations developed by the authors acting as technical consultants (i.e. expert witnesses) to the Public Prosecutor. In cases similar to that under study, it is very common that the collapse is disproportionate to the original cause. Hence, in the investigation, a clear distinction between the triggering cause and the causes of the damage evolution was made. The purpose of the investigations was not only to determine the causes of such a collapse but also to give the Public Prosecutor sufficient data to identify the parties responsible. The aim of the article is also to discuss the methodology used for the investigations and the relevant results in order to trace general criteria useful for forensic structural investigations.