I'm going to start by answering you in very broad terms. Denis Tougas will definitely be able to add to my answer and tell you about what goes on directly in the field.
It varies a great deal. Sometimes the UN force in the field can take part in and facilitate the investigations. The population wants the UN force to stay there and protect them. However, in many cases, those people can do nothing, have little training and cannot act. In those regions, their authority to intervene varies significantly. It really depends on the region itself and the MONUSCO force on site.