The electric field computed by numerically solving the one-dimensional Vlasov-Poisson system is used to calculate Lagrangian trajectories of particles in the wave-particle resonance region. The analysis of these trajectories shows that, when the initial amplitude of the electric field is above some threshold, two populations of particles are present: a first one located near the separatrix, which performs flights in the phase space and whose trajectories become ergodic and chaotic, and a second population of trapped particles, which displays a nonergodic dynamics. The complex, nonlinear interaction between these populations determines the oscillating long-time behavior of solutions.