40-year-old López Montero was once a soldier, but was soon recruited into the ranks of the United Self-Defense Forces of Colombia (AUC), becoming active mainly in Colombia's Eastern Plains. His relationship with another mid-level paramilitary commander, Pedro Oliveiro Guerrero, alias 'Cuchillo' dates back years. 

After Guerrero killed AUC warlord Miguel de Arroyave in 2004, Arroyave's army, known as the Centauros Bloc, split into two subgroups. López Montero joined one of these, which became known as the "Heroes del Llano" Bloc.