Germany’s air traffic control restricted flights at the airport shortly after 10 p.m. (2000 GMT) on Thursday and then halted them altogether, airport operators said in a statement. Seventeen flights were unable to take off, affecting almost 3,000 passengers, while 15 arriving flights were diverted to three other airports in Germany and one in Vienna, Austria.

Flights in and out of the airport resumed at 5 a.m. (0300 GMT), said Stefan Bayer, a spokesperson for Germany’s federal police at Munich airport. Authorities were not immediately able to provide any information about who was responsible for the overflights.

A statement from the airport early Friday said there had been “several drone sightings,” without elaborating. Bayer said it wasn’t immediately clear how many drones might have been involved. He said police, airline employees and “regular people around the airport” were among witnesses who reported the drone sightings.

After the closure of the runways, federal police deployed helicopters and other means to try to track down the drones, but no signs of them could be found, Bayer said.

Hundreds of stranded passengers spent the night in cots set up in terminals or were taken to hotels, and blankets, drinks and snacks were distributed to them, the German news agency dpa reported.

Drones were spotted overnight in Belgium

The incident was the latest in a series of incidents of mysterious drone sightings over airports as well as other critical infrastructure sites in several European Union member countries. A drone incident in Oslo, the capital of Norway, which is a NATO member but not part of the EU, also affected flights there late last month.

In Belgium, several drones were spotted overnight above a military base near the German border, Defense Minister Theo Francken told Le Soir newspaper.

The minister did not confirm how many drones were flying in the vicinity of the Elsenborn military base — which serves mainly as an army training facility with a firing range – just after midnight. Belgian public broadcaster VRT said that 15 drones were spotted near the base, which is roughly 600 kilometers (about 375 miles) from Munich.

Francken underlined that the nature of the flights was “suspicious and unknown,” Le Soir said. A defense ministry investigation is ongoing.

‘Anybody’ could be behind the flyovers

It wasn’t immediately clear who has been behind the flyovers, but European authorities have expressed concerns Russia could be behind them. Russian authorities have rejected claims of involvement in recent drone incidents in Denmark.

Hans-Christian Mathiesen, vice president of defense programs at Sky-Watch, a Danish maker of a fixed-wing combat drone that is being used in Ukraine, said “it could be anybody” who could carry out a drone flyover like the one at Munich airport.

“If you have a drone, you can always fly it into restricted airspace and disrupt activity. So everything from boys not thinking about what they’re doing — just fooling around — to someone that is doing it with a purpose: Criminal organizations, state actors, you name it,” said Mathiesen, whose company is involved in the fast-evolving drone ecosystem.

A state actor could disrupt activities and examine responses “with a minimal level of effort,” he said.

Officials in Russia and close ally Belarus acknowledged last month that some drones used as part of Russia’s war in Ukraine had entered the territory of EU and NATO member Poland, prompting a scramble by Polish and NATO allies in which fighter jets were deployed to shoot them down.

The drone overflights were a major focus of a summit of EU and European leaders in Copenhagen, Denmark, this week. Authorities have vowed to step up measures to minimize and thwart the threat posed by drones.

A Russian tanker is back at sea

Separately, a Russia-linked oil tanker that authorities in France detained — which had been suspected of possible involvement in the drone incursions over Denmark — was back at sea on Friday. The ship-tracking website Marine Traffic showed the ship heading southwest from the French Atlantic coast where it was detained and apparently bound for the Suez Canal.

A thorough search by French Navy commandos that boarded the ship found no drones, no drone-launching equipment and no evidence that drones had taken off from the vessel, according to an official with knowledge of the investigation who spoke on condition of anonymity because they weren’t authorized to discuss it publicly.

The tanker’s name has changed several times and it’s now known as “Pushpa” or “Boracay.” Its route from a Russian oil terminal into the Atlantic took it past the coast of Denmark.

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Associated Press writers Lorne Cook in Brussels and John Leicester in Le Pecq, France, contributed to this report.