This article was reproduced with the kind permission
of the British Broadcasting Corporation
1 November 2007, 04:59 GMT
Specialists from the Health and Safety Executive (HSE) will sift through evidence at the Petroplus Coryton refinery, near Stanford le Hope, Essex.
The explosion when the fire broke out on Wednesday caused buildings to shake 14 miles (22km) away.
Managers at the refinery said the fire started after a leak of petroleum gas.
Nobody was hurt in the fire, which broke out at about 1130 GMT.
Ten fire engines and seven special units were called to the refinery, which is about 30 miles (48km) from London, along the Thames Estuary.
Essex Fire Service said the blaze began on the ground floor of a tower.
HSE inspectors had been at the site on Tuesday, the day before the fire, carrying out routine checks.
A spokesman said the checks were not directly linked to the area where the fire broke out.
Black smoke could be seen miles away. Picture by Lister Appleton
At the height of the blaze, a huge column of smoke could be seen from about 20 miles (32km) away.
The Swiss oil company Petroplus bought the Coryton Refinery from BP in May 2007 for £714.6m.
The firm operates three other refineries in Europe, including one on Teesside.
The Coryton site was originally commissioned in 1953, has a refining capacity of 10 million tonnes a year, and is responsible for loading about 700 tankers a day to meet 22% of the UK's forecourt demand.
Jon Barden, refinery manager, said on Wednesday: "The rest of the refinery is running normally and we'll work with our people to ensure normal production is resumed as soon as possible.
"There will be no shortages, we can maintain the deliveries that we plan to our customers."
The HSE said its experts would be examining whether there were any similarities between the fire and the huge explosion at an oil storage depot at Buncefield in Hemel Hempstead, Hertfordshire, in December 2005.