Unmanned drones flew over Paris, for the second night in a row, raising questions as well as security concerns in a city that was the target of a terrorist attack last month. At least five sightings of the banned airborne vehicles were reported over central Paris by witnesses over the last two days.

Nearly three drones were spotted near the Invalides military museum, the Place de la Concorde and two of the old city gates on Tuesday, police officials said, according to reports. On the previous night, five drones were spotted in parts of the city, including near the Eiffel Tower and above the U.S. embassy near the Place de la Concorde.

“Is it a game? Scouting for future operations? The investigation will show us," a Parisian police official said Tuesday, according to Agence France-Presse (AFP).

The drone sightings will now be investigated by a team of 10 members from the city’s police force. Authorities were first warned about the drones in October, when state-run power company EDF filed a complaint after witnessing about 10 small drones flying over seven nuclear power plants. In November, about 20 drones were seen flying over nuclear plants, but the people responsible have not yet been found.

France has banned small civilian drones from sensitive areas such as nuclear facilities, which have a no-fly zone in a 1.6-mile radius and a height of 1,000 meters, AFP reported.

France has been on high alert since one of the worst terror attacks on French soil in January claimed the lives of 17 people. And, while authorities are unsure of the drones’ origins or purpose, security officials are worried about a possible security threat, BBC reported.