The fire broke out last Tuesday at about 1pm creating huge plumes of smoke which could be seen as far away as Northampton.
Fire officers later reported the flames caused extensive damage and there was speculation that white goods, expanding in the heat, caught fire.
A spokeswoman for the fire service said the lead fire investigator has now completed their preliminary investigation.
She added that they are now awaiting the next stage of work, which will be in conjunction with investigators working on behalf of the insurers.
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Public Health England said ash was deposited several miles downwind of the fire - as far away as Kettering.
At its height, eight crews were in attendance and police closed roads.