We describe a set of seismological observations of the foreshock sequence preceding the 6 April 2009 Mw 6.3 L'Aquila earthquake. The dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock. Approaching the earthquake, we observed clear variations in the seismic wave propagation properties. The elastic properties of rocks in the fault region underwent a sharp change about a week before the earthquake. From our observations, we infer that a complex sequence of dilatancy-diffusion processes takes place and that fluids play a key role in the fault failure process.