A fire on an upper-floor apartment balcony in Fitchburg was extinguished by the building’s sprinkler system Monday afternoon, allowing residents to remain in their homes, fire officials said. Dane County Communications alerted Fitchburg Fire Rescue to a structure fire at 6270 Limestone Lane shortly after 3:30 p.m. Initial reports described flames outside the building. Firefighters arrived to find the fire already out. The apartment where the fire occurred sustained minor damage, while nearby units were protected, according to the department. No injuries were reported, and officials deemed the building safe for residents to occupy. Fitchburg police, FitchRona EMS, Verona Fire Department and Madison Fire Department also responded. Fitchburg fire investigators are investigating the cause.