London firefighters are working with police to investigate the cause of a huge fire at an electrical sub-station which shut down Heathrow Airport on Friday, a fire-fighting chief told reporters. Huge orange flames and plumes of black smoke shot into the sky around 11 p.m. (2300 GMT) on Thursday as a blaze engulfed the substation. "The fire involved a transformer comprising 25,000 litres of cooling oil fully alight," Jonathan Smith deputy commissioner at the London Fire Brigade.