Investigators say a failed steel vent plug was the cause of a $2.5-million fire at Regina's Co-op refinery last month.

The fire broke out in the asphalt processing area after extreme pressure and temperatures caused the plug to deteriorate.

The asphalt that came out was hot enough to ignite when it was exposed to air.

No one was injured in the incident. It was the third fire at the refinery in 16 months.