A fire broke out early Friday at a Cotati duplex that was under renovation, destroying the building.

The blaze appeared to have started in an attic around 4 a.m. and quickly spread to the rest of the unoccupied duplex on East School Street, Rancho Adobe Fire Battalion Chief Mike Weihman said. When firefighters arrived, part of the roof already had collapsed. Multiple drivers called 911 after seeing the fire from Highway 101, he said.

“The reports to 911 came from people on the highway who thought it was a wildfire,” Weihman said. “The glow reflected over the trees. They couldn’t tell what was burning.”

Rohnert Park firefighters were the first to arrive, Weihman said. Cal Fire, Petaluma and Gold Ridge fire departments also responded to the blaze, which caused about $200,000 in damage, he said.

The fire spared another unoccupied duplex and single rental unit on the property, which butts up against Highway 101.

The cause of the fire remains under investigation.