Firefighters responded around 4 a.m. to reports of a fire alarm and smoke at the complex on the 500 block of Baden Avenue, according to a news release from the South San Francisco Police Department. Upon arrival, firefighters found that one unit was entirely engulfed by flame. The rest of the building was evacuated. Firefighters then worked to extinguish the burning unit and put out the fire quickly, authorities said. Damage to the unit was limited.