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While it wasn’t immediately clear who was behind the flyover, Denmark’s prime minister and NATO’s secretary-general said that Russian involvement couldn’t be ruled out. And now Denmark — already on edge because of its proximity to Russia — will join a group of front-line countries Friday to discuss the European Union’s plans for a “drone wall.�

Danish Prime Minister Mette Frederiksen called it “the most serious attack on Danish critical infrastructure to date.�

NATO Secretary-General Mark Rutte said that it was “too early to say� whether Russia was involved, while Kremlin spokesman Dmitry Peskov rejected suggestions that Moscow could be involved.

“Every time we hear unfounded accusations,� he said in a call Tuesday with reporters, adding that “a party that takes a serious and responsible position mustn’t make such unfounded accusations again and again.�

‘A capable actor’

Officials chose not to shoot down the drones because the risk was too great because of the airport being full of passengers, the planes on the runways and nearby fuel depots, Jes Jespersen, senior police inspector of the Copenhagen Police, said during a news conference.

Jespersen called the operators “a capable actor� and said they seemed intent on showing off their skills and possibly practicing their techniques. The drones' lights reportedly turned on and off and appeared to engage in different flight patterns.

“It all indicates that you are not out to attack anyone, but you are out to show off and maybe to practice,� he said of the operators.

The two to three drones appeared to have flown many kilometers (miles) to reach the airport. Investigators are looking at how the drones reached the airport — whether it was by land or possibly by boat.

Flights at the airport resumed early Tuesday, though delays and cancellations continued throughout the day.

A drone in Norway

A drone incident the same evening at the airport in Oslo, Norway, forced all traffic to move to one runway, according to Norwegian broadcaster NRK. Traffic later returned to normal and it’s unclear who was responsible.

Jespersen said nothing immediately linked the Oslo and Copenhagen incidents, but officials would look into any potential ties.

London’s Gatwick Airport closed its runway for almost an hour after a drone was reported nearby. , more than 140,000 travelers were stranded or delayed during the Christmas season after dozens of drone sightings shut down Gatwick for parts of three consecutive days.

Heightened security concerns

Security concerns in northern Europe are heightened following growing Russian aggression. On Tuesday, NATO warned Russia that it would use all means to defend against any further breaches of its airspace after the downing earlier this month of and Estonia’s report of an last week.

“And here we see a clear pattern: Russia is testing the European borders, also probing our resolve and undermining our security throughout,� Anitta Hipper, European Commission spokesperson, said Tuesday.

The Sept. 10 incident in Poland was the since Russia launched a full-scale war on Ukraine on Feb. 24, 2022. It jolted leaders across Europe, raising questions about how prepared the alliance is against Russia.