Fish and other aquatic animals generate thrust by oscillating their fins and flukes creating a highly three-dimensional and complex wake that is characterized by spanwise compression, transverse expansion, wake breakdown, etc. Understanding of wake dynamics is important in order to design an efficient bio-inspired propulsion system. Lagrangian analysis using the finite-time Lyapunov exponent (FTLE) field was carried out on experimentally recorded three-dimensional data in the wake of an oscillating trapezoidal panel modeling a fish caudal fin in a Strouhal number range of 0.17 - 0.56. Behavior of the wake structure observed in the isosurfaces and contours in the three-dimensional FTLE field was compared with the measured forces generated by the panel from a numerical simulation. Occurrences of peaks and troughs of thrust correlated with the release of saddle points associated with the formation and the shedding of the spanwise vortices at the trailing edge. The location of vortex disintegration, which is observed further downstream, was observed to correspond with low momentum flow regions, bifurcated wake structure, and the merger of saddle points from two consecutive spanwise structures.