A fire which sparked a mass evacuation at Altnagelvin Hospital started in a small room where electrical equipment was being stored, the fire service has said.
More than 100 patients from seven wards were cleared from the Co Londonderry hospital after a blaze broke out on the roof of the tower block.
An investigation by Northern Ireland Fire and Rescue Service has determined the cause of fire as accidental.
An NIFRS spokeswoman said: "Although some additional forensic tests will need to be carried out and additional analysis conducted before the full fire investigation report is complete, the preliminary findings indicate that the fire started accidentally in a small room on the roof of the tower block, adjacent to the water storage tanks. This room contained some portable electrical equipment.
"The fire did not start in the fabric of the main building and was not caused by the fabric of the main building."
The spokeswoman said the investigators had faced difficult and challenging conditions when trying to establish their preliminary findings because of where the fire originated.
Most of the 120 evacuated patients were accommodated in other parts of the Altnagelvin Hospital or were moved to other appropriate health and care settings in Londonderry. One patient from the Fermanagh area was transferred to the South West Acute Hospital in Enniskillen.
Dozens of planned operations have been cancelled or postponed over the past week.
The Western Health Trust has also taken a decision not to put patients back into wards, eight, nine or 10 which are on the upper levels of the damaged building. Some staff at Altnagelvin had raised concerns about the state and safety of the tower block.
However, on Monday Stormont Health Minister Edwin Poots told the Assembly he did not believe the fire was linked to the fabric of the hospital building. The minister said Altnagelvin was safe to use.