Illegally increased fsi limit

Neighboring housing society (apartment) has illegally increased the permitted FSI limit which was granted by local authorities. The FSI increase permit was given to both buildings, ours and the neighboring one but they have illegally constructed more than the limit and now a mere 5-6 meter gap between both buildings. Whom should we approach? And what steps should we take? The building srtucture is around 3-4 years old.