A fog of petrol fumes and water vapour 200 yards wide moved across the Buncefield oil depot minutes before December's blasts, investigators say.
Devastation caused by the fires has delayed the investigation
But exactly what led to the cloud or caused it to ignite is not yet known.
The devastation from the blasts, which wrecked the depot in Hemel Hempstead, Hertfordshire, and caused major damage to homes, has held up investigators.
Officials have revealed they were investigating a small leak two or three weeks before the disaster.
The blasts injured 43 people, but no-one was killed.
The investigation, which was ordered jointly by the Health and Safety Executive (HSE) and the Environment Agency, has been described as the "most wide-ranging" of its kind since the Potters Bar rail crash inquiry in 2002.
Eyewitnesses have reported seeing a low-lying mist by one of the protective "bunds" surrounding petrol tanks at the west of site, near the neighbouring industrial estate, prior to the first blast on 11 December.
It was also captured on CCTV.
The fuel vapour had escaped from a bund, although it is not known whether over-filling or some form of leak was responsible, investigators said.
Taf Powell, the lead investigator at the Buncefield site, said: "This mixture flowed very quickly and ignited with the force we have seen."
He said it was "too difficult to say at this time" what caused the ignition, adding "there could have been a number of ignition sources".
Mr Powell's report, released on Tuesday, states: "The visible mist seen in the CCTV records and described by witnesses is assumed to arise from the evaporation of the more volatile fractions of an escape of fuel.
"The most plausible scenarios involve large-scale loss of containment of vessels or pipework within Bund A."
He adds: "There appear to have been several explosions but the exact sequence of events has not yet been established."
The HSE has also revealed it was investigating a similar, but smaller, leak at the depot two or three weeks before the blasts.
The safety body has notified more than 1,000 fuel storage sites, including 100 oil depots of the same type as Buncefield, of a safety and inspection drive.
But residents are angry that the HSE is investigating its own performance in the run-up to the blast, and are demanding a public inquiry.
Inquiry chairman Lord Newton of Braintree told a news conference in Hemel Hempstead: "The extensive damage to the site and the need to make it safe has unavoidably delayed the investigating team in gaining access and carrying out a thorough forensic examination of the scene."
Lord Newton has been joined in examining the blasts by Prof Dougal Drysdale, an authority on fire safety, and Dr Peter Baxter, a medical expert.
Environment Agency and HSE staff are also on the inquiry board.
The local authority for the Buncefield depot area - Dacorum Borough Council - said it welcomed the Health and Safety Executive's publication of the progress report.
Providing "considered conclusions for lasting solutions" is more important than the immediate demand for answers to what happened, it said.
The final findings of the inquiry will be made public.