French police launched an investigation on Wednesday, November 12, after an unidentified drone was spotted flying over a police station and a train carrying battle tanks in northeastern France, a police source said.

In recent months mystery drones have rattled nerves in Europe, with unmanned aerial vehicles flying over sensitive military and industrial sites, shutting down airports and raising concerns about the continent’s security. Suspicions have swirled over potential Russian involvement in drone activity in countries including Britain, Belgium and Denmark, with tension high as the war in Ukraine drags through its fourth year.

The latest sighting in France occurred over the central police station in the city of Mulhouse near the German border on Tuesday night, the police source said, adding that “at least two round trips” had been seen. Police officers on patrol noticed the drone flying at around 11 pm Tuesday and attempted to follow it to the nearby Gare du Nord train station, in an area not accessible to the public. A security guard made the same observation.

A convoy of Leclerc battle tanks returning from a military exercise was parked at the station, a military source said. Local authorities were not aware of the presence of the convoy transporting the tanks, the police source said. The origin of the drone has not been identified, added the source.

On Monday, an investigation was launched after an unauthorized drone flew over a gunpowder plant in the southwestern city of Bergerac, according to the authorities. In late September, drones were spotted over a military base in northeastern France, where Ukrainian soldiers underwent training.