A two-alarm fire at a Healdsburg commercial building Tuesday night caused more than $500,000 damage, fire officials said.

The Grove Street fire damaged two businesses, Encore Events and Aaction Rents, said Healdsburg Fire Chief Steve Adams.

Nearly 50 firefighters from seven agencies responded to the fire call.

Investigators with the Sonoma County fire task force concluded Wednesday that the blaze was caused by improperly stored oil-base stain rags, said Adams.

"It's not uncommon for oil rags to be the cause," said Adams, who noted that a 2011 three-alarm fire at Nu Forest Products in 2011 also was traced to oil rags.

The damaged businesses contained a large quantity of combustible items that created difficulties for firefighters.

It took several hours to contain the fire, but much of the building and contents were saved, including computers, office equipment and rental items.

Calls to 911 began at 8:12 p.m. with numerous reports of heavy smoke and flames. The first engines and crews from Healdsburg arrived three minutes later.

It was drill night for Healdsburg firefighters, who were gathered at the station for training.

When the calls came in, 16 local firefighters, paid staff and volunteers, were able to respond, said Linda Collister, Healdsburg's division chief who ran the fire.

Healdsburg was joined by Cloverdale, Geyserville and Windsor firefighters.

At 8:21 p.m. a second alarm was called, getting more engines and firefighters rolling from Rincon Valley, Santa Rosa and Forestville, as well as additional Windsor equipment.

Firefighters initially were on an offensive attack, working from inside and on the roof.

But at 8:26 p.m. a portion of the roof collapsed, making those spots too dangerous.

Firefighters were alerted to get out and work from the exterior.

When the effort turned in their favor, they returned to an offensive effort, allowing them to save some of the huge building and contents, said Collister.