A liquid petroleum gas appliance in a Castlerock flat where two Newtownabbey teenagers died last week was defective, the NI Health and Safety Executive (HSENI) has said.
Aaron Davidson and Neil McFerran, both 18, died in Tunnel Brae Court apartment last Tuesday following a suspected carbon monoxide leak.
Their friend, Matthew Gaw, survived.
HSENI said it was continuing to carry out checks in other premises in Castlerock and Coleraine.
It said in order to determine the level of risk and to minimise any further risks, the executive was working with its industry stakeholders to contact LPG customers who may be affected.
As an additional precaution, HSENI is advising members of the public who had gas work carried out in their home or premises by George Brown, trading as George Brown of 'The Calor Shop' in Coleraine, to switch off all gas appliances and contact HSENI's emergency helpline number - 0800 0320 121.
The HSENI emergency helpline will operate from 0800 BST until midnight.
The HSENI said the 'The Calor Shop' is an entirely separate legal entity to Calor Gas (NI) Limited.
As large volumes of calls are expected in the first 48 hours, HSENI asks that only members of the public with specific emergency safety concerns use this helpline.
In a statement the safety body also issued some general advice and said anyone displaying any of the following symptoms - headache, dizziness, nausea, breathlessness, collapse, loss of consciousness, should seek medical help immediately.
They should also contact their local Gas Safe registered engineer and request a full gas safety check.
The HSENI also advises installing an audible carbon monoxide (CO) alarm as a further precautionary measure.
The funeral service for Aaron Davidson took place at The Church of the Ascension, Newtownabbey on Tuesday.
A funeral service for Neil McFerran was held at Newtownabbey Methodist Mission in Rathcoole on Saturday.