The busiest Nordic region airport Copenhagen Airport reopened early Tuesday after drone sightings halted all take-offs and landings for nearly four hours.

Norway’s Oslo airport resumed operations after temporarily shutting its airspace due to similar drone incident.

“The police have launched an intensive investigation to determine the nature of these drones”, said Copenhagen Police Deputy Assistant Commissioner Jakob Hansen. He noted that the drones had vanished and were not recovered.

Authorities in Denmark and Norway are collaborating to investigate any potential connection between the two incidents.

Oslo Airport’s airspace was reopened by 3:22am (0122 GMT) after being closed since midnight (2200 GMT) due to drone observations, which led to the diversion of all flights to nearby airports.

Earlier on Monday, Danish police reported sightings of two or three large drones near Copenhagen Airport, prompting the closure of the facility to all traffic. Operations were halted at 8:26pm (1826GMT) on Monday, leading to approximately 50 flights being diverted, according to flight service Flight Radar.

After reopening, Copenhagen Airport warned of ongoing delays and some cancelled departures, urging passengers to check with their airlines.

The shutdowns come amid a series of disruptions at European airports in recent days, including a cyberattack last Friday that affected check-in and boarding systems at London’s Heathrow, as well as Berlin and Brussel’s airport.

In 2018, drone sightings over Gatwick Airport near London caused significant travel disruptions, stranding tens of thousands of passengers during the busy holiday season.