Firefighters say the blaze which devastated a Halifax town centre mill started in or around the sauna area of a gym in the building.
A spokesperson for West Yorkshire Fire and Rescue Service said the most likely cause of the fire at Greenwood's Mill in Square Road was the sauna.
She added: "Due to the level of damage our expert fire investigators have been unable to definitively establish the exact cause."
Firefighters were called to the mill at 3.10am on May 29.
Several local businesses based at the building have been left desperately seeking temporary new premises.