Four warhead-carrying drones strayed into Finnish territory in separate incidents during the spring.Police image of a drone which crashed near the city of Kouvola on 28 March. Image: PoliisiYle News14:17Finnish authorities have concluded their investigation into separate incidents during the spring when drones carrying warheads strayed into Finland.The incidents were jointly investigated by the National Bureau of Investigation (NBI) and the Finnish Border Guard.In a statement, the authorities said that all four drones originated from Ukraine, were similar models and were manufactured for military use. Each drone was carrying explosives, the statement confirmed.The drones were discovered during March in Kouvola, Luumäki and near Parikkala, and in April near the town of Iitti.Finnish authorities believe that one of the drones exploded while in the air, while the other three were detonated in controlled explosions shortly after they were discovered.The Finnish Border Guard investigated the incidents as violations of Finnish territory, while the NBI probed the potential charge of aggravated endangerment."The preliminary investigations have not revealed any evidence that would have justified continuing the pursuit of criminal liability," the press release stated, adding that the cases are now closed.Finland not a targetIn its statement, the NBI and the Finnish Border Guard emphasised that Finland was not the intended target in any of the incidents where the drones strayed into Finnish territory.Instead, the devices were likely "hijacked" in mid-air while flying towards targets in Russia, and redirected westward.UK newspaper The Telegraph uncovered evidence in the spring that a transmitter in the Russian exclave of Kaliningrad uses GPS jamming and spoofing to redirect Ukraine's drones away from Russia and towards the airspace of Nato member states.The investigation by Finnish authorities also noted the impact of the prevailing weather conditions as a key factor."The wind direction at the time of the incidents was almost directly from the south. These factors are assessed to have affected the aircraft's navigation and their deviation from their original route," the press release stated.None of the four drones that ended up in Finnish territory caused any personal injury or damage to property.Ukrainian President Volodymyr Zelensky issued an apology to Finland in the aftermath of the drone find near Kouvola and emphasised that Ukraine was not targeting Finland under any circumstances.Yle News' All Points North podcast heard from UK security expert Keir Giles about the phenomenon of drones straying into Finnish airspace. You can listen to the episode via this embedded player, on Yle Areena, via Apple, Spotify or wherever you get your podcasts.Inside Finland’s drone scareInside Finland’s drone scare