Advances in RFID based sensor technologies has been used in applications which requires the tracking of assets, products and individuals. The recording of such movements is captured in a trajectory database and can be analyzed for the monitoring of abnormal events. In this paper, we describe a system called InViTA for analyzing abnormal events from spatio-temporal trajectories captured during an of ce evacuation after an explosion. InViTA utilizes a trajectory representation scheme and extract the features to derive a set of rules that label each person's trajectory as belonging to a suspect, witness, or victim, etc. We run the system on the of ce evacuation data provided in VAST 2008 challenge and obtain comparable results with that obtained from visualization and human analysis. The system includes a user-friendly graphical interface for parameter tuning and intuitive result analysis. Keywords-representation scheme, trajectory classi cation, abnormal events