An employee of drone manufacturer Atlas Aerospace, Dmitry Belus tests a drone in the Latvian capital of Riga on Feb. 1, 2023. (AP file photo)

BERLIN (AP) — Mysterious drone flights over the airspace of European Union member countries in recent weeks have alarmed the public and elected officials.

Intrusions into NATO’s airspace, some of them blamed on Russia, reached an unprecedented scale last month. Some European officials described the incidents as Moscow testing NATO’s response, which raised questions about how prepared the alliance is against Russia.

On Sept. 10, a swarm of Russian drones flew into Poland’s airspace, forcing NATO aircraft to scramble to intercept them and shoot down some of the devices. It was the first direct encounter between NATO and Moscow since Russia launched its all-out war on Ukraine on Feb. 24, 2022.

Days later, NATO jets escorted three Russian warplanes out of Estonia’s airspace.

Since then, flyovers have occurred near airports, military installations and critical infrastructure, among other locations, elsewhere on the continent and prompted European defense ministers to agree to develop a “drone wall” along their borders to better detect, track and intercept drones violating Europe’s airspace.

Russia has been blamed for some of them, but denies that anything was done on purpose or that it played a role. European authorities haven’t released much detail about the drone intrusions, with some not acknowledging the overflights publicly until days later.

On Thursday, however, Russian military planes briefly violated Lithuania’s airspace in what the Lithuanian president called a blatant breach of the territorial integrity of his EU and NATO-member country.

Here’s what we know about some of the recent sightings:

Airport closures

Germany’s Munich Airport was shut down twice earlier this month due to drone sightings.

In Denmark, drones flew over Copenhagen Airport on Sept. 22, causing a major disruption to air traffic in and out of Scandinavia’s largest airport.

Danish Prime Minister Mette Frederiksen said that Russian involvement couldn’t be ruled out, calling it “the most serious attack on Danish critical infrastructure to date.”

Meanwhile, a report of a drone sighting occurred the same evening at Norway’s Oslo Airport, forcing all traffic to one runway. Authorities were investigating whether there was a link between the two major airports.

Military sites

Drones also flew over four smaller Danish airports between Sept. 24 and Sept. 25, including two that serve as military bases. Danish Defense Minister Troels Lund Poulsen said that a “professional” actor was likely behind the “systematic flights.”

In the same time frame, several Danish media outlets reported that one or more drones were seen near or above the Karup Air Base, which is Denmark’s biggest military base.

The defense ministry refused to confirm the sighting at Karup or elsewhere, and said that “for reasons of operational security and the ongoing investigation, the Defense Command Denmark does not wish to elaborate further on drone sightings.”

Critical infrastructure in Germany

In Germany, authorities investigated claims that unidentified drones may have spied on critical infrastructure in the northern state of Schleswig-Holstein. Multiple drones were spotted on Sept. 25 over a power plant in the state capital of Kiel, as well as near a university hospital and a shipyard in the port city, according to a Der Spiegel report.

Regional Interior Minister Sabine Sütterlin-Waack told a committee of the state parliament that flying objects of “various types and sizes” had been spotted. Chief Public Prosecutor Stephanie Gropp said an investigation was underway.