Could be both:
With its close proximity to the M1 motorway, the Buncefield oil depot has become one of the UK's major distribution terminals. Reported to have a storage capacity of 150,000 tonnes, Buncefield holds massive stocks of not only oil and petrol but also the aviation fuel kerosene, used to supply airports across the region including Heathrow and Luton just 10 miles away. The depot itself first came into use in 1968 but for the last 15 years has been run as a joint venture by oil giants Total and Texaco. It is also used by the other leading petrol firms BP and Shell. http://www.timesonline.co.uk/TGD/tgdBreakingNewsDisplay/0,,2,00.html#0
Initial reports said witnesses heard a plane going overhead shortly before the first explosions - but police were quick to dismiss that suggestion, insisting the incident was an "accident".