A soldier killed three of his superiors at a military base in south-western Colombia in an act of violence that the Army blamed on the killer\'s mental problems Friday, dpa reported. The incident happened Thursday in the village of Puerto Bello in the Cauca province. The rank-and-file soldier opened fire with his regulation rifle, killing a sergeant and two corporals. The Army said in a statement that the event was being investigated, but noted that the preliminary probe indicates that the soldier \'lost his self-control.\' The authorities ruled out that he might be an infiltrated rebel.