The possibility of injecting single electrons into one-dimensional ballistic conductors is at the basis of the new field of electron quantum optics. In this presentation I will focus on the single electron injection process into interacting topological quantum Hall or quantum spin Hall edge states. The unavoidable presence of electron-electron interactions strongly affects the time evolution of charge and energy wave packets, leading to their fractionalization. We elucidate the dynamics of fractionalization by calculating the nonequilibrium electron distribution function in the momentum and energy domains after the injection. The results show how fractionalization occurs via the creation of particle-hole pairs by the injected electron.