The investigation into the deaths of two teenagers poisoned by leaking fumes in Northern Ireland is now a criminal investigation.
Health and safety experts are helping police with their inquiries into the work of a gas shop owner involved in fitting appliances at more than 100 properties.
Two boys were found dead at an apartment overlooking the beach at Castlerock, Co Londonderry, last week. A third survived the carbon monoxide fumes.
Dermot Breen, deputy chief executive of the Health and Safety Executive for Northern Ireland, said a robust and thorough criminal investigation was under way following the tragic deaths.
He added: "We owe it to them and their families to ensure that we do not say or do anything at this stage that may prejudice this ongoing investigation. We have deployed the necessary teams with technical knowledge to either assist with the investigation, or identify gas safety issues that require correction."
Police on Tuesday night searched the premises in Coleraine, Co Derry, of George Brown, a gas shop owner, who is at the centre of the inquiry.
In the aftermath of the deaths of Aaron Davidson and Neil McFerran, both 18, from Newtownabbey, Co Antrim, customers who had appliances fitted by his firm were warned to disconnect them. It later emerged that an appliance inside the apartment at Tunnel Brae Court where the boys were found dead was defective.
The health and safety executive has dealt with more than 500 emergency calls from worried customers since they announced a major alert after the tragedy on Tuesday of last week.
Meanwhile, the Northern Ireland Finance minister Sammy Wilson said here should be no "kneejerk" reaction to the deaths and called for a considered approach to legislation which could make fitting carbon monoxide alarms in some houses compulsory. After a meeting with Trade and Investment Minister Arlene Foster, he said: "We want to make sure that if we do decide to do something that it is effective, we achieve the objectives that we need."
Mr Wilson and Ms Foster will examine if enforcing the installation of the potentially life-saving detectors in new builds, rented accommodation and businesses is feasible. Other jurisdictions that considered the option did not proceed because of problems identifying an industry standard device.