Knowledge of the dynamic behavior of complex buildings subjected to near‐fault earthquakes may be enriched by valuable information obtained through rapid onsite dynamic testing to aid in the design of appropriate retrofitting interventions. Through a case study, the article demonstrates the enhancement in comprehension of the structural behavior obtained by means of a 2‐day testing campaign conducted on a complex building of the Engineering Faculty, Edifice A, which was heavily damaged in the 2009 L'Aquila earthquake. The onsite testing was carried out with a network of 13 accelerometers opportunely located to identify the dynamic characteristics of the structure by means of ambient noise‐induced vibration. Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification (SSI) output‐only procedures were both used to identify the main modal parameters of two substructures of the building. The modal characteristics were used to update a finite element model representing small amplitude vibrations of the damaged structures. The direct comparison of the identified modal features with finite element models, in which damaged member locations are determined by onsite visual observations, has permitted identification of a model representative of the structural behavior of the building in the immediate postearthquake conditions.