Nine people awakened by fire in a west Sonoma County duplex escaped unharmed early Thursday morning, Graton fire officials said.

The fire was contained to a garage area between the housing units, thanks in large part to the prompt arrival of Gold Ridge and Sebastopol fire crews dispatched as mutual aid, Graton Chief Terry Davis said.

"It was a real good stop," Davis said.

The fire broke out shortly before 2:30 a.m. on Mill Station Road near Barlow Lane.

Heavy duty batteries left on a sofa stored in one of the garage units are suspected of causing the fire, which quickly ran up an interior wall and began to spread through the attic, Davis and Assistant Chief Bill Bullard said.

The fire was checked before it reached the living spaces on either end of the building.