MILAN — Bomb threats targeted more than a dozen flights in Europe on Thursday, leading to a slew of emergency landings and military escorts in some cases, authorities said. No hazardous or suspicious materials have been found in what a British official called “malicious” hoaxes.

Several similar threats targeted planes earlier this week, prompting searches and diversions. Europe has experienced a slew of other disruptions recently, such as attacks on supermarket refrigeration systems, drone incursions and mysterious triggers to alert apps.

While fake bomb threats are relatively common, it is unusual to see so many in a single day. They come at a time of increased geopolitical tensions and warnings from authorities that Russia may be ramping up so-called hybrid threats in Europe.

It wasn’t known whether this week’s threats were linked, who sent them or what they hoped to achieve.

An email containing the threat to Rome-bound planes was sent on behalf of a group that described itself as anti-feminist, according to a copy provided by an airport official and reviewed by The Associated Press. An official from another European airport said the threat they received also came from a group calling itself anti-feminist, and used some of the same language. It wasn’t clear if the same group sent the two threats. Both officials spoke on condition of anonymity because they weren’t authorized to share the information.

At Rome’s airport, travelers whose flights weren’t affected seemed to take the threats in stride.

“As of now, I am not scared of being in an airport or getting on board a plane,” said Alessia Strada, a 19-year-old Italian returning from Kenya. “Probably this idea I have about traveling in Europe will change in the near future, but let’s see how it goes.”

Alex Langenkamper, a 38-year-old German headed to Munich, said “crazy people” exist and will in the future.

Meanwhile, Michael Romano, a 54-year-old traveler headed to Colombia, expressed concern about the world moving toward war.

“These things never used to happen here in Europe,” he said. “This is all new, so it is a little bit scary.”

Jacob Funk Kirkegaard, senior fellow at the Bruegel think tank in Brussels, said the threats were “stress-testing” how European governments and airport officials respond, and whether their responses were coordinated.

The idea behind the threats “appears to be to sow chaos, uncertainty, about the reliability of air traffic across Europe” and could have “cascading effects” — like economic damage and costs related to inspection, investigation and cleaning aircraft.

Joji Waites, who heads flight safety at the British Airline Pilots’ Association, said the industry has put in place “proved mechanisms” for dealing with such threats, including a team of assessors to grade the level of threat in a red-amber-green color-coding system, quick notification of flight crews, and the evacuation of planes on the ground.

Each flight disruption could cost airlines as much as $250,000, on a par with the possible costs of dealing with an unruly passenger, said Philip Baum, a visiting professor of aviation security at Coventry University in England.

“And that’s because you might end up with the crew going out of hours. You might need replacement crew. You have got passengers that miss their flight connections,” Baum said. “It knocks everything out of sync for the rest of the day for the airline in question.”

Military escorts cost money, too, which would be borne by governments rather than airlines, he added.

Britain’s transport secretary, Heidi Alexander, told British broadcasters that flights across Europe have been hit by “malicious hoax threats.”

European Union Transport Commissioner Apostolos Tzitzikostas told reporters in Dublin that it was “too early to draw conclusions on whether the reported incidents are linked.”

“National investigations remain ongoing,” he said, adding that an EU information-sharing mechanism on aviation security had been activated.

An Aer Lingus flight from Dublin to Rome was diverted to a military air base in France after receiving a threat, French Prime Minister Sébastien Lecornu’s office said. The flight landed safely at the base in Evreux, which was under high alert, and military bomb disposal teams were examining the plane, the office said.

Lecornu ordered warplanes to intercept the plane because the pilot “didn’t cooperate as expected with the authorities,” his office said. It declined to comment further.

Hours after the flight landed, the plane was sitting on the tarmac with a Red Cross vehicle nearby while explosive experts with dogs examined it.

An investigation was opened by French aviation security authorities. The regional prosecutor, Remi Coutin, said in a statement that the pilot and co-pilot were questioned about what happened.

Italian air force fighter jets escorted a Finnair flight that departed Helsinki bound for Rome after a threat was received by email, the air force said in a statement. Croatia’s defense ministry said one of its fighter planes first intercepted the commercial jet in Croatian airspace before handing it off to the Italians.

Finnair said that two of its flights, including the one that received the military escort and landed safely in Rome, “faced unfounded hoax bomb scares.” The other flight, operated by Jettime from Helsinki to Vienna, landed safely at its destination, and was expected to return to service after an inspection, Finnair said.

Three Ryanair flights bound for Sofia, Bulgaria, meanwhile, also received military escorts and made emergency landings after bomb threats, officials said. Two of the flights — one from Copenhagen, Denmark, and the other from Madrid — landed safely in Sofia, airport officials told reporters. The third, from London, was diverted and landed safely in Plovdiv, Bulgaria’s second-largest city, Plovdiv Airport CEO Krassimir Peshev said.

Several flights across Europe received bomb threats earlier this week, officials said.

The flights included ones operated by Polish airline LOT, Scandinavian Airlines, Wizz Air and Ryanair. The hoax threats came after a plane headed for Bulgaria was diverted to Hungary upon receiving a similar bomb threat while in the air. No one was injured.

A slew of other flights were disrupted Thursday. Among those incidents:

— Czech police said searches of four planes that landed in Prague after being targeted by bomb threats turned up nothing. They haven’t given details on where the flights originated, and said they took precautions despite not finding the threats credible.

— A LOT Polish Airlines flight from Warsaw to Brussels landed at its destination, where inspections found no hazardous or suspicious items, the airline said.

— A Wizz Air flight from Milan to Bucharest landed safely in the Romanian capital following a bomb threat, airport spokesman Teo Postelnicu said. Checks turned up no bomb.

— A flight to Warsaw landed in Vienna after Polish authorities received a threatening email, said Stefan Loidl, a police spokesperson in the Lower Austria province. Nothing was found and the plane continued to Warsaw. He said there was also an abstract threat to Vienna Airport concerning four other flights, which he declined to identify, but operations weren’t disrupted.

– Air Serbia said a flight from Belgrade to Bucharest received a bomb threat that turned out to be a hoax. The plane landed in Bucharest, underwent checks and resumed operating.

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Charlton reported from Paris and Biller from Rome. Associated Press writers throughout Europe contributed to this report.