Two restaurants have been ordered to close by health officials inspecting gas appliances around the area where two teenage friends died from carbon monoxide poisoning.
The owners of the premises in Portstewart, Co Londonderry, and Rasharkin, Co Antrim, pulled down the shutters as inspectors continued to question the gas fitter who installed the heater suspected of killing 18-year-old school pals Aaron Davidson and Neil McFerran in the nearby town of Castlerock.
The development came as Stormont ministers prepared to meet to discuss whether carbon monoxide detectors should be compulsory in new builds in Northern Ireland.
Aaron and Neil, from Newtownabbey outside Belfast, had been spending time at a seaside apartment awaiting their exam results when they were overcome by poisonous fumes last week. A third teenager, Matthew McGaw, also 18, survived the leak.
The Health and Safety Executive (HSENI) found that a liquefied petroleum gas (LPG) appliance inside a flat at Tunnel Brae Court had not been working properly. The heater was fitted by the Calor Shop in Coleraine, which is run by George Brown.
All businesses and householders who had their appliances fitted by Mr Brown had subsequently been urged to switch off their supply and call the HSENI.
It is understood the proprietors of the Ice House restaurant in Portstewart and a chip shop in Rasharkin contacted the HSENI - two of nearly 500 people to ring an emergency helpline since it was set up on Monday.
When asked about the restaurant closures, a spokeswoman from HSENI said: "HSENI and gas supply companies have been active in the Castlerock and Coleraine areas to make safe a number of homes and premises as part of their ongoing emergency response."
Earlier, Mr Brown passed his condolences to the teenagers' families and pledged his co-operation with investigators. A solicitor for Mr Brown released a statement: "He thanks all those friends and customers who have contacted him offering messages of support, but would ask that all thoughts and prayers be with the Davidson and McFerran families at this time."
Northern Ireland Enterprise Minister Arlene Foster and Finance Minister Sammy Wilson have called for compulsory carbon monoxide alarms.