Investigators are looking into claims by a tanker driver that he triggered the Buncefield oil blast near London by flicking his engine cut-out switch while there was a vapour leak from the plant.
The man's friend told The Sun newspaper: "There was some sort of vapour leakage, like a dense mist. It was 'heavy' - forming in the air and dropping to the ground.
"It seemed to be coming from behind his tanker as he couldn't see a hole.
"My colleague ran to his tanker, flipped the switch and waited. His engine cut out, then 'boom!' He does not know whether he caused it but he blames himself."
A spokeswoman for Hertfordshire Police confirmed that the driver had given detectives a similar account.
He said: "He is one of the key witnesses and we have spoken to him but whether that is what caused it or not is not something that we know at this stage."
Police, the fire service and the Health and Safety Executive, are looking into the cause of the blast. Meanwhile, a bitter war of words erupted last night over claims that firefighters were "woefully prepared" to fight the fire.
Hertfordshire fire chief Roy Wilsher reacted furiously to the Fire Brigades Union's allegations as it was announced the inferno near Hemel Hempstead had been defeated after a 60-hour battle.
Praising his officers as "magnificent" he poured scorn on the FBU's criticism and accused it of "pure opportunism". He said: "I'm disgusted that the FBU have sought to make political capital out of this incident and to mislead the public, when they should be applauding the actions of firefighters who have performed nothing less than a miracle."
The last of the blazing oil tanks at the Hertfordshire site, engulfed on Sunday by a series of explosions, was extinguished at 5pm yesterday, although a number of small fires were still burning. The FBU accused Hertfordshire Fire Authority of only being able to deal with minor oil fires and having almost no capacity to tackle the blaze with foam.
It called for a public inquiry into the fire.
PA