On September 18, 2004, a 4.6 mbLg earthquake occurred in the western Pyrenees, with an epicenter located approximately 4 km from the Itoiz reservoir dam. We study the aftershock sequence of this earthquake and compare it with others that occurred in the same area in order to evaluate whether this might be a case of reservoir induced seismicity triggered by impounding of water. In order to do so, we first decluster the catalog by means of a link algorithm whose distance is based on the ETAS model. We then analyze the different clusters obtained in terms of their fractal dimension and the properties of the complex network formed by the individual earthquakes belonging to these clusters. We conclude that there are properties of the cluster representing the September 18, 2004 earthquake that are distinctively different from the properties of other clusters found in the same region. This could be an indicator of a different seismic source, potentially produced by the Itoiz dam impoundment.