A major investigation into the blasts at the Buncefield oil depot has begun.
The fire burned for days and caused extensive damage
The Health and Safety Executive (HSE) said it would be the most wide-ranging investigation of its kind since the Potters Bar train derailment in 2002.
Justin McCracken, of the HSE, said he hoped to produce an interim report "within weeks".
Prohibition notices have been issued at the Hertfordshire site to prevent firms from clearing up until experts say it is safe for the people and environment.
The Health and Safety Commission, which oversees the HSE's work, served the notices on Hertfordshire Oil Storage Ltd (a joint venture between Texaco and Total) and British Pipeline Agency Ltd.
The commission said being issued with a notice did not suggest the firms had done anything wrong.
Police have said they had found nothing "malicious" in the incident, which happened in Hemel Hempstead on 11 December.
The HSE is now in charge of the full investigation.
At a press conference, deputy chief executive Mr McCracken revealed the HSE had reviewed and approved fire measures in the weeks leading up to the blasts.
He said in the "specific" areas they looked at "nothing came out which caused us undue concern".
According to the Health and Safety Commission, the inquiry would consider issues such as the factors leading up to the blaze, what ignited it, and the root causes.
The commission's chairman, Bill Callaghan, said the inquiry would also "make recommendations for future action to ensure the effective management and regulation of major action risk" at sites similar to Buncefield.
The report into the incident will be made public
An initial report would be prepared as soon as "main facts" had been established, and made public unless there were legal obstacles, he said, adding that it would be chaired by an independent figure.
Mr Callaghan said his HSE colleagues investigated many incidents each year, and it was clear Buncefield was "no routine incident".
He said: "It's considered by the HSE and the Environment Agency to be a significant event.
"In regard to the severity of the incident, the degree of public concern, and the enforcing authority's previous involvement with the duty holders at the depot, it is therefore one that merits a major incident investigation."
An HSE spokesman said the prohibition notices were served on two of the three firms based at the depot as a way of controlling the recovery operation after such a major incident.
"We were very fortunate that there were no deaths in this incident.
"Our aim is to ensure that in recovering the site that work is not done that would inadvertently create a risk to people, or the wider environment," said the spokesman.