In August 2015 more than 300 dolphin calves were stranding on the Bulgarian seacoast. Data were collected from witnesses, organized by us in a network, using IT online services and other communications. Further these data were formatted in GIS context. Using the numerical model MOTHY and the estimated exact hour of death of dolphin carcasses, were chosen 28 specific cases were chosen. We simulated 252 possible trajectories for 25 days, with more than 50 000 coordinate points. These raw data from the model were organized in a specific geodatabase. Then, adavanced GIS analysis were used to locate the place and exact time of death of dolphin’s calves, back in time. The results point a place in Black sea, 35 km north of Snake Island that was the most probable place where the dolphin’s calves died around 25 July 2015. Suspected reason of death, determined by different ways, was the usage of illegal gillnets for turbot. Further, historical data of ship’s traffic were analyzed (with active AIS) to determine possible offender/s. As only small percent of carcasses reach the coast in the investigated area due to specific currents in the Western Black Sea, it is suspected that the incident concern thousands of dead dolphin calves. The significance in its investigation is apparent for preventing new similar incidents. The applied new advanced methods and workflow can be used to investigate different types of past time accidents in the sea.