Dr Edward Haughey, his colleague Declan Small and two pilots named as Captain Carl Dickerson and Captain Lee Hoyle all died when the Agusta Westland AW139 came down in Norfolk on 13 March.A special bulletin published by the Air Accidents Investigation Branch (AAIB) on Thursday did not identify any technical fault but pointed to the "dense fog" that night as a focal point for inquiry.It said: "AAIB investigation to date has not identified any technical malfunction which might account for the accident. The investigation continues, with the aim of identifying any technical matters of relevance, as well as focussing on flight in degraded visual environments."The helicopter had been scheduled to leave Gillingham Hall for Coventry Airport at 6.30pm but the passengers had not been ready to depart until around 7.20pm.The report went on: "By this time, night had fallen and dense fog had developed; witnesses described visibility in the order of tens of metres."The investigation is ongoing and a final report will be published in due course."Lord Ballyedmond, who was 70, was the founder of the world-renowned Norbrook Laboratories and was one of Northern Ireland's richest men.Mourners at his funeral in Co Down last month heard his death described as a "huge loss".