An airport in northern Denmark was forced to close because of unauthorized drones in its airspace, police said early Thursday, the second drone-caused shutdown of a Danish airport this week as Europe grapples with a rise in cyberattacks and suspected Russian airspace violations.

“More than one drone” was seen flying near Aalborg airport, which is also an active military base, at about 9.45 p.m. local time Wednesday, and remained nearby until just before 1 a.m. Thursday, police said, according to Reuters. The drones are no longer in the area, police confirmed.

Police have not yet determined who is behind the latest incident but are investigating several theories, Denmark’s National Police Commissioner Thorkild Fogde told CNN.

The drone sighting comes two days after sightings of two or three large drones halted all takeoffs and landings for nearly four hours at the airport in the capital Copenhagen, in what the prime minister called a “serious attack on Danish critical infrastructure” according to Reuters.

It also follows a major cyberattack at a provider of check-in and boarding systems over the weekend, which disrupted operations at several of Europe’s busiest airports, including London’s Heathrow.

It’s unclear if the drones near Aalborg and Copenhagen are linked, but the pattern was similar, Fogde said. Both are cases of “unauthorized drones over-flying the airport area, breaking airport security, violating the airspace.”

In a separate incident this week, Norwegian authorities were forced to close Oslo Airport for around three hours after a drone sighting, causing more travel chaos.

In comments to Danish public broadcaster DR on Tuesday, Prime Minister Mette Frederiksen suggested Russia could be behind the disruption in Copenhagen, which she linked to other drone incidents in Poland and Romania, Reuters reported.

“I certainly cannot deny in any way that it is Russia,” Frederiksen told the broadcaster.

Kremlin spokesperson Dmitry Peskov called suggestions of Russian involvement “unfounded.”

Europe on high alert

Europe has been on high alert after a series of Russian breaches of NATO airspace. Last week, NATO intercepted three Russian jets that entered Estonian airspace, forcing them to flee, Tallinn’s foreign ministry and an alliance spokesperson said.

Three Russian MiG-31 fighter jets entered Estonian airspace over the Gulf of Finland without permission and remained there for a total of 12 minutes, the Estonian foreign ministry said.

Italian F-35 fighters that were stationed in Estonia as part of NATO’s Eastern Sentry operation, in addition to Swedish and Finnish aircraft, responded to the intrusion, NATO Allied Command Operations headquarters said. Estonian Prime Minister Kristen Michal said the Russian jets were subsequently “forced to flee.”

Russia later denied its jets had entered Estonian airspace, insisting the flight was carried out “in strict accordance with international rules” and “without violating the borders of other countries.”

In separate incidents earlier this month, NATO fighter jets shot down what the alliance said were Russian drones which had violated Polish and Romanian airspace during an attack on neighboring Ukraine.

The operation marked the first time that shots were fired by NATO since Russia’s full-scale invasion of Ukraine, which began in February 2022. The military alliance denounced Moscow’s “absolutely dangerous” behavior, and pledged to beef up defenses on its eastern flank.

Days later, Romania said a Russian drone breached its air space, prompting Bucharest to scramble fighter jets.

The two F-16 jets came close to downing the drone but pilots decided not to open fire after assessing the collateral risks.

After the Copenhagen incident Tuesday, Ursula von der Leyen, president of the European Commission, said on social media that “while the facts are still being established, it is clear we are witnessing a pattern of persistent contestation at our borders.”

“Our critical infrastructure is at risk,” she said. “And Europe will respond to this threat with strength and determination.” (CNN)