The Mw 6.1 (6 April 2009) L’Aquila (Italy) earthquake occurred in one of the most seismically active areas of central Italy and was preceded by a three-month-long foreshock period. Thanks to recordings by a regional permanent network, we derive for the first time P- and S-wave velocity tomographic models of a major fault prone to an imminent main shock. Close to the Mw 6.1 hypocenter, we observe high Vp (>6.8 km/s) and high Vp/Vs (>1.9) consistent with thick dolomitic volumes filled with fluids sealed by impermeable anhydritic layers. Significant changes in velocities defined by time-lapse imaging during the foreshock period suggest rapid fluid migration through the locked fault zone. The complex positive feedback between fluid pressure buildup and hydrofracturing of the dolomitic reservoir, testified by foreshock production, eventually provoked the catastrophic coseismic breaching of the fault seal. Our results show that foreshock time-lapse tomography provides clues on the preparatory phase of a large normal-faulting earthquake.