A fire which had been burning at a tyre depot near Londonderry since the weekend has been put out.
Firefighters arriving to tackle the blaze on Sunday night found four separate fires across the one-acre site at Eglinton.
It is thought as many as half a million tyres were being stored there.
The fire service said tackling the blaze had been a "mammoth task" and the cost of the operation is expected to run into tens of thousands of pounds.
An investigation will now begin into whether the fire was started deliberately.
It is understood the depot was storing tyres illegally.
The site, which was sub-let, did not have the licence required by environmental law to store a large number of tyres.
The BBC understands that before the fire broke out, a criminal investigation was already being held into why so many tyres were being stored there.
A medical packaging company based near the tyre depot fire in Londonderry has said "having smoke in their building would have been catastrophic".
Perfecseal, which makes makes medical packaging for pacemakers and cardio vascular stents, had to prepare to seal the building for fear of contamination.
Keith McCracken, director of European operations at Perfecseal, the company was "very conscious of the risk."
"We were very fortunate that we weren't affected at all, we monitored wind conditions in the event that the wind changed to blow smoke towards the plant," Mr McCracken said.
"We ramped up with equipment on Monday morning to allow us to hermetically seal the building to prevent any ingressive smoke whatsoever.
"Having smoke in the building would have meant having to close the building for weeks to clean it," he added.
"So we've been extremely fortunate and with the collaboration the fire service we managed this situation satisfactorily."