Using the activation-relaxation technique (ART), we study the nature of relaxation events in a binary Lennard-Jones system above and below the glass transition temperature (T_g). ART generates trajectories with almost identical efficiency at all temperature, thus avoiding the exponential slowing down below T_g and providing extensive sampling everywhere. Comparing these runs, we find that the number of atoms involved in an event decreases strongly with temperature. In particular, while in the supercooled liquid activated events are collective, involving on average thirty atoms or more, events below T_g involve mostly single atoms and produce minimal disturbance of the local environment. These results confirm the interpretation and the generality of recent NMR results by Tang et al (Nature 402, 160 (1999)).