Fire broke out at a College Avenue law office in Santa Rosa early Monday, causing substantial damage, according to a fire official.
The pre-dawn fire initially appeared suspicious but further investigation ruled out possible arson, said Santa Rosa Battalion Chief Mike Jones.
Callers reported the fire at 2:24 a.m., bringing a total of 23 firefighters to the flat-roofed, two-story building at 717 College near Humboldt Street.
The first engines arrived at 2:28 a.m., from the nearby Sonoma Avenue station.
Firefighters went up on the roof to cut holes and let out heat and smoke. Others forced open a front door to get in.
They found one room burning and signs there was more fire burning, unseen, in spaces between the ceiling and the second floor.
Thermal imaging cameras helped firefighters seek out hidden pockets of fire, which took time to find and extinguish, Jones said.
He estimated $200,000 damage to the building and $50,000 to contents.
The building contains offices for multiple lawyers.
The fire initially appeared suspicious as firefighters were told homeless people had been seen near the building prior to the fire and firefighters found some people had been sleeping alongside the building.
But there were no signs of a break-in and fire officials ruled out a suspicious start, said Jones.
The fire started inside and the cause remained under investigation Monday.
Santa Rosa sent five fire engines, a fire truck, two fire officials and a fire prevention officer to the call. They were aided by a Rincon Valley fire air truck crew.