An investigation is under way after four contractors were killed in an explosion and fire at a Pembrokeshire oil refinery. Another worker suffered serious burns in the blast at the Chevron refinery…Greg Hanggi, refinery general manager, said…that Chevron would take every step possible to determine the series of events leading to the tragic incident and ensure that any lessons learnt from it would be learned…Earlier Ch Supt Gwyn Thomas of Dyfed-Powys Police said officers were informing the victims' families and an investigation was under way into what appeared to be a "tragic industrial incident”…There have been previous incidents at the refinery. In June 2001 a fire was caused by a hydrogen leak…In March 2003 there was a fire in a specialist catalytic cracking unit, and in September that year two people were taken to hospital after fire in a steam plant. [also refers to Texaco, Valero Energy]