Three companies are facing potentially unlimited fines after they were found guilty of health and safety breaches in connection with the explosion at the Buncefield oil depot.
TAV Engineering Ltd, of Guildford, Surrey, was found guilty of failing to protect workers and members of the public, the Health and Safety Executive (HSE) said.
Motherwell Control Systems 2003 Ltd, which is in voluntary liquidation, was found guilty of the same charge on Wednesday at St Albans Crown Court.
Hertfordshire Oil Storage Limited (HOSL) was also found guilty of failing to prevent major accidents and limit their effects - a verdict which could be reported for the first time on Friday.
The company also pleaded guilty to causing pollution to enter controlled waters underlying the vicinity around Buncefield, contrary to the Water Resources Act, an HSE spokeswoman said.
The verdicts follow a joint prosecution by the HSE and the Environment Agency which the bodies described as the "biggest and most complex criminal inquiry" they had worked on together.
The destruction at the Hertfordshire depot came after a massive vapour cloud ignited when 250,000 litres of petrol leaked from one of its tanks.
The blast on December 11 2005, widely thought to be the largest explosion in peacetime Europe, measured 2.4 on the Richter scale and could be heard 125 miles away.
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