Refineries are plants in which due to the activities of storage, transport and refining of oil and oil derivatives, the danger of fire outbreaks and explosions is a daily danger. Refineries have internal documents on the basis of which fires and explosions are prevented, but also on the basis of which they act in the event of them in order to save the refinery's personnel as soon as possible, and then the plants and all material value. On June 20, 2011, at 10:48 a.m., a fire broke out in the Sisak Oil Refinery. The fire broke out in the pipeline of the main pipelines of crude oil, gasoline, slopa, heating oil, gas oils and propane butane gas. These pipelines connect process plants to KP - 6, KP - 7 and storage space through manipulation via pumping stations. The fire occurred when the process plant KP - 7 was in the phase of movement. The raw material that was supposed to reach the plant came from a converted tank A - 3, which was previously in the function of another. In this paper, I will try to investigate the cause of the fire from the sources available to me, guided by investigative questions and the beginnings of the investigation. Six workers were injured, one of them sustained severe bodily injuries.