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,” adding that no suspects had been identified.

Axar.az reports, citing Reuters, the airports in Copenhagen and Oslo, the two busiest in the Nordic region, were shut for hours after drones were observed in their airspace late on Monday, leaving tens of thousands of passengers stranded as flights were diverted.

Copenhagen Airport was closed for four hours when two or three large drones were seen flying in its immediate vicinity, officials said, while the Oslo Airport was closed for three hours following two sightings, according to local police.