WALNUT CREEK, Calif. -- Fire investigators continued to search for clues Monday as to what started a four-alarm fire that destroyed a lumberyard in Walnut Creek over the weekend, Contra Costa County Fire Protection District spokeswoman Emily Hopkins said.

Federal investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were working alongside investigators from the county fire department, which is common for large fires, Hopkins said.

"They have a number of highly trained investigators and we really appreciate their help," Hopkins said.

She said she expected fire investigators to remain on the scene through Tuesday.

The fire was first 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 at one point.

No injuries were reported, but Interim Fire Chief John Ross said the building on the property is a complete loss.

A preliminary estimate put damages at about $5 million, Hopkins said.

A two-alarm fire that damaged property at Piedmont Lumber's yard in Pittsburg last August was ruled suspicious, but no suspects were ever identified, Hopkins said.