Copenhagen Airport, the busiest in the Nordic region, reopened early on Tuesday after drone sightings halted all take-offs and landings for nearly four hours. Meanwhile, Norway's Oslo Airport shut its airspace due to a drone sighting.
"The police have launched an intensive investigation to determine what kind of drones these are," Copenhagen Police Deputy Assistant Commissioner Jakob Hansen told reporters. "The drones have disappeared and we have not taken any of them," he added.
Hansen said authorities in Denmark and Norway would cooperate to determine whether there was a link between the two incidents.
Oslo Airport shut its airspace from midnight local time (2200 GMT) due to a drone observation, a spokesperson for Norwegian airport operator Avinor said in an emailed statement, adding that all flights were diverted to the nearest airport.
Danish police said earlier on Monday that two or three large drones had been spotted flying near Copenhagen's airport, forcing it to close to all traffic.
Flights grounded in Norway, Denmark
The airport halted operations at 8:26 p.m. (1826 GMT) on Monday, according to flight tracking service FlightRadar. Around 50 flights were diverted to alternate airports, FlightRadar said on X.
After it reopened, Copenhagen Airport announced on X that delays and some cancellations would persist and urged passengers to check with their airlines.
The airport shutdowns came after a string of disruptions at European airports in recent days.
A cyberattack last Friday disrupted check-in and boarding systems supplied by Collins Aerospace, a unit of RTX RTX.N, affecting operations at London's Heathrow Airport and the airports in Berlin and Brussels. Over the weekend and into Monday, the fallout continued to snarl travel across the region.
In 2018, drone sightings over the runway at Gatwick near London stranded tens of thousands of passengers and disrupted hundreds of flights at the height of the holiday season.
Drones flown by a "capable operator"
Danish police said on Tuesday that drones that shut the country's main airport on Monday appeared to have been flown by "a capable operator" seeking to demonstrate certain abilities, adding that no suspects had been identified.
"We have concluded that this was what we would call a capable operator," Danish police Chief Superintendent Jens Jespersen told reporters on Tuesday, referring to the drones observed in Copenhagen.
"It's an actor who has the capabilities, the will and the tools to show off in this way," Jespersen said, adding that it was too early to say if the incidents in Denmark and Norway were linked.
Jespersen said the drones in Denmark came from several different directions, turning their lights on and off, before eventually disappearing after several hours.