Published Thursday, 29 November 2012
Fire crews at Altnagelvin Hospital after a fire broke out last week. (© UTV)
Around 120 patients were evacuated from seven wards when the blaze broke out in a small room on the roof of the tower block on Friday.
A pipe then burst on the roof, causing tanks to leak water, which cascaded onto the floors below.
A Fire Service spokesperson said the investigators "had to work in difficult and challenging conditions to produce their preliminary findings".
"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."
But a spokesperson added, "The fire did not start in the fabric of the main building and was not caused by the fabric of the main building."
It is the second fire in the tower block this month, prompting concerns over the safety of the building.
Further forensic tests are expected to take place before the Fire Investigation Report is complete.