Fire officers in Londonderry today refused to rule out arson as the cause of a blaze at Altnagelvin Hospital.
The fire broke out last night close to where thousands of medical records are held. Senior medical staff said today it was fortunate the fire had not occurred elsewhere.
Almost half a million pounds worth of damage was caused to the new renal unit at Altnagelvin in an arson attack this time last year.
Hospital services manager Raymond Jackson said: "Sunday night's fire was in the old outpatients' corridor which is not used for patients, it is mostly offices and medical records use it although they were not damaged.
"There was quite a lot of damage to one office but it is structurally OK."
Fire and Rescue officers were today due to return to the scene of the blaze today to determine the cause.
Crescent Link station commander Kevin Lynch said that arson could not be ruled out. He said the alarm was raised through an automatic alert system.
"When we got there we found the fire was in a locked-up office in the old day care unit," he said.
"We forced entry and found what was a significant fire in one of the offices and firefighters using breathing apparatus extinguished it.
"We are keeping an open mind as we are not sure what happened but we will be going back today to investigate."