Arson investigators with the fire district are continuing to work with Walnut Creek police and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives on the investigation.
Fire district spokeswoman Emily Hopkins said investigators are not saying whether they believe the fire is suspicious.
The fire was reported at 1:44 p.m. Saturday at Piedmont Lumber at 2860 N. Main St. It grew into a four-alarm blaze, bringing about 70 firefighters and other personnel to the site.
No injuries were reported, but Interim Fire Chief John Ross said the building on the property was a complete loss.
A preliminary estimate put the damage at about $5 million, Hopkins said.
A two-alarm fire that damaged property at Piedmont Lumber's yard in Pittsburg in August was ruled suspicious, but no suspects were ever identified in that case, Hopkins said.
That fire started in an exterior, enclosed lumber storage rack. Flames spread to another, larger storage rack, which then radiated heat to a business next door.
According to Hopkins, most of the damage caused by the Pittsburg fire was to the neighboring business, Wally's Rentals. Piedmont Lumber's main offices were not damaged.
Anyone with information about the Saturday fire is asked to call the fire district's arson tip line at (866) 50-ARSON.