The application of vibration-based damage detection using ambient measurements requires an accurate and reliable estimation of the modal parameters. This study presents a comprehensive statistical analysis of the identified modal properties of a full scale five-story reinforced concrete building - built and tested on the NEES-UCSD shake table - using recorded ambient vibration data. The building specimen was tested under base-isolated and fixed-base conditions. On April 6, 2012, when the building was isolated at its base, a dense accelerometer array was deployed on the structure and ambient vibration data were collected continuously until May 18, 2012, three days after the last seismic test in the fixed-base configuration. A state-of-the-art method of operational modal analysis was used to automatically identify, from the recorded ambient vibration data, the modal properties of the fixed-base building, including natural frequencies, damping ratios and mode shapes. A statistical analysis of the identified modal parameters is performed to investigate the statistical variability and accuracy of the system identification results. The variability of the identified modal parameters due to environmental conditions is also investigated.