Sunday's blaze in Milton, Stoke-on-Trent, centred on a building containing recycled plastics.
The Environment Agency had previously said Hanbury Plastics ,the site operator, had never held an environmental permit.
The operation to tackle the fire has been scaled back but it is still burning in places.
See more stories from across Stoke and Staffordshire here
The blaze at the site, in Leek New Road, Milton, started around 08:00 GMT on 5 February.
The EA said the site's owners had been issued with legal notices to clear waste plastics.
Staffordshire Fire Service said it is helping police with its investigation.
Investigation Officer Ian Broadhead said: "We have worked closely with Staffordshire Police during the investigation and concluded that the fire was started deliberately."
A fire at a recycling plant which was storing illegal waste was started deliberately, the fire service said.