I urge you to read in details the indicators and information available on this case on pages 32 and 33 of the report. You're right, the case has not gone before the courts. There has not been a satisfactory investigation and that's why we cannot say for sure that he was indeed assassinated by the army. I cannot prove that to you.
The fact that the investigation has not taken place is a problem in itself which was identified by numerous organizations. Canada and the UN in particular have recognized that impunity in Colombia is a problem. What is not being debated is the responsibility for the death of this person. The Colombian army has indicated that he is a guerillero who died on the battlefield.