Firefighters spent about seven hours battling a blaze which consumed 400 hay bales yesterday.
Two fire appliances from Inverurie, one from Oldmeldrum and a water carrier from Turriff were called to the incident at Mill O’Burns Farm, between Rothienorman and Fyvie, at about 2.15am.
Initially the fire was not thought to be suspicious, though today (September 8) police said they were treating it as a case of willful fire raising.
A Police Scotland spokesperson said: “We are following a positive line of inquiry.”
Thirteen firefighters were called to the scene and worked until 9am to bring the blaze under control.
The bales were situated in a field directly by the side of the unclassified Fyvie to Cross of Jackson road and the incident caused thick smoke to drift along the route.
The road was closed until about 9am with morning traffic redirected.
Nothing was left of the field’s fencing yesterday and the ground where the bales had been sitting was left blackened and scorched with roadside bushes reduced to ash.
Aberdeenshire fire and rescue station manager, David Meldrum, said it had been hard for firefighters to move the burning bales and get to the seat of the fire.
Mr Meldrum added: “We got the call at 2.10am in the morning and the last appliance left just after 8.30pm.
“They had to deal with the smoke going over the road, that obviously would have been a hazard for early morning traffic and then they had to move a large amount of bales to actually get into the heart of the fire.
“The firefighters worked hard throughout the night to bring the incident under control.
“When you’re dealing with these bales the fires can be deep-seated, a lot of the work involves just moving the bales to get into the heart of where the fire is burning.”