The investigation into the fire that has devastated Mourne Park House in Kilkeel is to begin on Friday, according to Northern Ireland Fire and Rescue Service.
Three wings of the house have been saved and remain safe for human habitation but three floor levels in the main building have collapsed, leaving it unsafe for entry.
At the height of the blaze, which began on Saturday morning, 80 firefighters were involved in trying tried to bring it under control, but one appliance now remains and the site is monitored twice a day by a supervisory officer.
Area commander John Allen said: "At this moment, three floor levels have collapsed down on to the ground floor and small pockets of rubble area are burning within the structure.
"The building is not safe for firefighters to enter. There are three storey walls and no cross support so it would be reckless to put firefighters into it."
Mr Allen said it is anticipated that the investigation will begin on Friday, at which point a decision will be made over which organisation will be the lead agency.