High‐resolution tomography from the 2009 L'Aquila extensional seismic sequence has shown that the Mw 6.1 main shock and most of the aftershocks occurred within a high velocity body (6.6 ≤ Vp ≤ 6.8 km/s), located between depths of 3 and 12 km. The nature of the high‐Vp body has remained speculative, although exhumed mafic deep crustal and upper mantle rocks (serpentinites) have been favored. We used 3D magnetic anomaly modeling to investigate the plausibility of these favored sources for the L'Aquila body. The modeling does not support the presence of high‐velocity serpentinites with a 30–50% serpentinization degree and gabbros. Accordingly, we conclude that the high‐Vp body may represent non‐magnetic upper Triassic and possibly lower Liassic dolomites that have been drilled in neighboring wells for 2–4 km. This conclusion is also consistent with the lack of a coherent gravity anomaly for the body. We speculate that ultra‐thick Triassic dolomites reaching a thickness of 8 km may have been deposited in syntectonic wedges formed at the northern margin of the Ionian Sea, where oceanic spreading occurred in mid‐late Triassic times.