L’articolo appare sull’International Journal of Forensic Engineering (IJFE), Vol. 2, No. 2, 2014 a firma di L. Fiorentini (TECSA), L. Marmo e N. Piccinini (Politecnico di Torino). Abstract: paper deals with the identification of the cause of an explosion in a hollow space positioned below a residential building as the result of welding operations. On field investigation clearly demonstrated that the explosion was due to the ignition of a flammable atmosphere in the basement of the building. Along the trial two hypotheses were brought to the judge: (a) explosion of an air-petrol mixture (b) explosion of an air-LPG mixture. Forensic engineering techniques have been applied in order to assess which hypothesis was the correct one, on the basis of the observed post-accident scene. Modelling of the vapour diffusion, the natural ventilation and the permeation of LPG through the soil showed that possibility (b) was by far the most likely.