A storage tank at the Buncefield oil depot was overflowing for more than 40 minutes before it exploded, causing a 32-hour inferno, a report has said.
Fuel was piped into the tank for 11 hours before the blasts on 11 December.
The Health and Safety Executive said that at 0520 GMT the tank was full, but gauges and safety devices did not work and 41 minutes later the tank exploded.
Some families were left homeless and 43 people were injured in the blasts in Hemel Hempstead, Hertfordshire.
But investigators will not look into shortcomings in the plant's design and operation "in order to avoid prejudicing future legal considerations".
Vapour cloud
The inquiry, overseen by Lord Newton of Braintree, found that two separate safety devices failed.
Lord Newton said: "The first was a float - a level gauge - which got stuck and therefore didn't rise and register as the tank continued to fill.
BUNCEFIELD REPORT
"And then it did fill, and just before that happened, an alarm should have been triggered - a so-called high-level alarm - and that doesn't appear to have operated either."
The overflow caused a huge vapour cloud to build up, which was then ignited by a spark.
This is the third progress report from the HSE and Environment Agency investigation, charged with finding out the causes and the environmental impact of the explosions.
"The evidence obtained to date supports the initial conclusions regarding the physical process and the mechanisms that led to the explosions, while not yet understanding why the main explosion was so violent," the report stated.
Hemel Hempstead's Conservative MP Mike Penning has called for a public inquiry into the blasts.
He said the progress report moved the inquiry along "only fractionally" and he expressed concern over investigations conducted "behind closed doors".
The MP added that businesses had to relocate away from the area and the government had not delivered the help it had promised.
Many people were left homeless by the blaze, and Margaret Kingston from local charity the Dacorum Community trust said she had had more than 350 requests for help.
"For the people who moved out, many are now coming back into their homes, but they're still facing a lot of problems."
She said some people had lost their jobs and were "really struggling".
Retired farmer Mike Hewitt has not yet been able to move back to his home.
But Mr Hewitt, 71, said he was confident new safety measures meant the chances of such a fire happening again "will be very remote".
"We have got to be optimistic and expect things to improve, but I cannot see them committing so many tanks again on such a small site.
"They've got to control it, there is an awful lot of tanks on the site."
Thick clouds
More than 20 tanks caught alight during the blaze, which raged for almost two days and was described as the largest in peace-time Europe.
"On behalf of the industry I would apologise for all of the disruption that the incident at Buncefield caused"
It threw up thick black clouds of smoke which were seen for miles around.
Chris Hunt, of the UK Petroleum Industry Association, said the industry as a whole apologised for the incident.
He said the industry was sorry for "all of the disruption that the incident caused to the community, both in terms of damage to homes and businesses, and to the environment".
The investigation, the biggest health and safety inquiry since the Potters Bar rail disaster in 2002, began at the end of January.
The first progress report dealt with the response to the incident, the second with the environmental impact of the explosion.
HOW TANK 912 OVERFLOWED
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