Fire investigators say the cause of a huge fire last month at an Abbotsford, B.C., mushroom farm was accidental.

The Aug. 27 fire at Avina Fresh Mushrooms near 283rd St. and 58th Avenue destroyed two composting barns and caused an estimated $4 million in damage, Insp. Spencer Clark of Abbotsford Fire Rescue Service told on Tuesday.

Clark said investigators believe one way the fire could have started is that a hay bale spontaneously ignited from a combination of heat and excessive moisture. The other possible scenario is that a piece of construction equipment sparked the blaze.

The fire started outside the buildings, he said.

Clark said investigators may have a clearer picture within a few weeks.

The company was under a completely different type of investigation last year.

According to a news release issued by the Department of Fisheries and Oceans in November 2008, a B.C. provincial court found that Avina Fresh Mushrooms Inc. and Avina Fresh Produce Ltd. permitted the deposit of mushroom farming waste into a tributary to Nathan Creek that is frequented by fish.

The court also found that Avina Fresh Mushrooms Inc. and H.Q. Mushroom Farm Ltd. carried out mushroom farming that caused harm and destruction of fish habitat in a tributary to Nathan Creek.

As a result, $90,000 in fines were issued, according to the press release.

Calls to Avina Fresh Mushrooms were not immediately returned on Tuesday.