The crimes were shocking even for a country hardened by decades of internal conflict.

U.N. investigators found that Colombian troops had killed hundreds of innocent civilians for no apparent reason other than to boost rebel body counts. Typically, the victims were lured to their deaths with job promises, then dressed in military fatigues and registered as guerrillas slain in combat.

Five years after the scandal broke, only about one-sixth of the soldiers accused have gone to trial or pleaded guilty. In all, authorities are investigating some 3,900 cases of alleged extrajudicial killings involving security force members.

And human rights activists say they are afraid a new law pushed through Congress by President Juan Manuel Santos will make it even harder to pursue those responsible, particularly senior officers.