A source detective system has been developed to estimate unknown source regions and release rates of radionuclides released into the air from covert nuclear activities and accidents. This system is composed of trajectory, atmospheric dispersion, and source term estimation models. Simulated results were compared with the measurements of a field tracer experiment performed at the Yeonggwang nuclear power plant in Korea in May 1996. Two trajectories among five computed backward trajectories moved toward the original release point, and the comparative results contained some error due to single operation of the backward trajectory model. An atmospheric dispersion model was used to minimize the error of the trajectory model and to improve the accuracy of the source detective system. The results generated by the trajectory and atmospheric dispersion models together agreed better with the measurements than those obtained using the trajectory model alone.