The case involved an incident in late August, when a Russian border guard transport plane entered Finnish airspace near Porvoo without permission.
The Russian Antonov 72 transport was said to be en route from Kaliningrad to Moscow, when it penetrated four kilometres into Finnish airspace, the Coast Guard said in a release.
Air traffic controllers notified the cockpit that the plane had entered Finnish airspace without authorisation, but later checks revealed that the flight plan involved a route that took the aircraft into Finnish skies.
Pilots of two Finnish Hornet jets scrambled to investigate the aircraft instructed the Russian pilot to leave Finnish airspace, and he later complied.
The Finnish Meteorological Institute confirmed that there had been cumulonimbus clouds in the area at altitudes of between four and six kilometers. However they weren't particularly large and there were no records of lightning over the Gulf of Finland at the time.
Finnish authorities confirmed three cases of Russian planes entering Finnish airspace without authorisation in August. At the time Defence Minister Carl Haglund said he didn’t believe that Russia was able to offer a credible explanation for the incidents.