Drones were observed over four airports across Denmark, causing one of them to close for hours, police said today, after a similar incident this week prompted Copenhagen Airport to shut.

Drones were seen at the airports in Aalborg, Esbjerg, Sonderborg, and at the Skrydstrup airbase before leaving on their own, police said.

Aalborg Airport, located in northern Denmark and one of the country’s biggest after Copenhagen, was shut down before reopening several hours later.

“It was not possible to take down the drones, which flew over a very large area over a couple of hours. At this time, we have not apprehended the drone operators either,” North Jutland chief police inspector Jesper Bojgaard Madsen said in a statement about the incident in Aalborg.

South Jutland police said they had “received several reports of drone activity at the airports in Esbjerg, Sonderborg and Skrydstrup”.

The Esbjerg and Sonderborg airports were not closed as no flights were scheduled there until tonight NZT.

Police there said the drones “flew with lights and were observed from the ground, but it has not yet been clarified what type of drones they are ... or what the motive is”.

Police said they were “investigating broadly in co-operation with [Danish intelligence service] PET and the Armed Forces to clarify the circumstances”.

Prime Minister Mette Frederiksen said yesterday that the country had seen its “most serious attack on Danish critical infrastructure”, after large drones flew over Copenhagen Airport and prompted its closure for several hours.

-Agence France-Presse