The Millennium Forum in Derry was evacuated on Wednesday evening after carbon monoxide was detected inside the building.
Approximately 1,000 people were evacuated from the building as a precautionary measure after the alarm was raised shortly before 7:30p.m.
Two Northern Ireland Fire and Rescue Service (N.I.F.R.S.) fire appliances from Northland Road Fire Station and three from Crescent Link Fire Station attended the scene.
Fire-fighters wearing breathing apparatus entered the building and using a gas monitor confirmed the presence of carbon monoxide in part of the building.
The source of the carbon monoxide was identified and the area ventilated.
The incident was dealt with at 10:05p.m.