The four-alarm fire broke out at about 1:45 p.m. on March 13 at Piedmont Lumber and Mill Company at 2860 N. Main St.
When firefighters arrived, they found a large portion of the lumberyard engulfed in flames and several nearby buildings threatened, fire officials said.
Firefighters controlled the blaze shortly after 6 p.m., but had to stay at the site overnight to monitor it for flare-ups.
Fire investigators were able to determine the area where the fire started, but could not pinpoint a specific ignition source, Fire Investigator Vic Massenkoff said.
He did say, however, that he found no indication that the fire was intentionally set.
Investigators confirmed that an arson fire that had been set along the exterior of a storage shed at Piedmont Lumber's facility in Pittsburg in August 2009 was unrelated to the fire at the Walnut Creek facility, Massenkoff said.
That fire only damaged two exterior sheds belonging to Piedmont Lumber. The majority of the damage was to adjacent businesses, Massenkoff said.
The fire in Walnut Creek, however, destroyed the facility and damage was estimated to be more than $6 million. The property is currently for sale, according to the fire district.
Walnut Creek police and the U.S. Department of Justice's Bureau of Alcohol, Tobacco, Firearms and Explosives assisted in the investigation.