MOSCOW — European officials on Monday were scrambling to respond to the forced landing in Belarus of a Ryanair passenger flight, as new details emerged about a brazen operation by the country’s strongman leader to arrest a dissident Belarusian journalist traveling on board.
Sofia Sapega, the girlfriend of the arrested journalist, Roman Protasevich, was also detained when the Ryanair flight from Athens to Vilnius, Lithuania, landed in Minsk on Sunday after a bogus bomb threat, her university in the Lithuanian capital said.
Ms. Sapega, a Russian citizen, was detained at the Minsk airport along with Mr. Protasevich under “groundless and made-up conditions,” the European Humanities University in Vilnius said in a statement demanding her release. There was no word Monday morning from the Belarusian authorities on their whereabouts.
The Boeing 737-800 was allowed to take off for Vilnius after about seven hours on the ground in Minsk on Sunday. But aside from Ms. Sapega and Mr. Protasevich, several other passengers also stayed behind, the police commissioner of Lithuania told The New York Times.
Michael O’Leary, the chief executive of Irish-based Ryanair, said those passengers may have been agents of the Belarusian intelligence service, which is still known by its Soviet-era initials.
“We believe there were some K.G.B. agents offloaded at the airport as well,” Mr. O’Leary told Irish radio on Monday. “This was a case of state-sponsored hijacking.”
Mr. O’Leary said Ryanair was in the process of debriefing its crew and that the European Union and NATO were “dealing with” the situation. The Lithuanian police said they had opened a criminal investigation, on suspicion of hijacking and kidnapping.
The Lithuanian government called for Belarusian airspace to be closed to international flights in response to what it called a hijacking “by military force.” The main carrier based in neighboring Latvia, airBaltic, said on Monday that it had already decided to avoid Belarus airspace “until the situation becomes clearer or a decision is issued by the authorities.”
At least six people who boarded the Ryanair flight in Athens were not on the plane when it finally arrived in Vilnius, Lithuania, the police commissioner of Lithuania said on Monday.
Among the six were the dissident journalist Roman Protasevich and his girlfriend, Sofia Sapega, who were both detained.
The identities of the other four people who apparently got off the plane after it was forced to land in Minsk, Belarus, on Sunday are not publicly known — and are being investigated by the Lithuanian police.
“What I know is that six people did not arrive in Vilnius,” Renatas Pozela, Lithuania’s police commissioner general, said in a telephone interview with The New York Times.
Further details, he said, “are being looked into as part of the criminal investigation.”
The Lithuanian police opened a hijacking and kidnapping investigation into the forced landing of the plane, and they questioned the pilots after they landed in Vilnius on Sunday evening, Mr. Pozela said.
Police investigators are interviewing the passengers this week, he said.
“The pilots were the priority,” Mr. Pozela said. “We wanted to hear their stories. How did they see the situation? What did they do? Were there other planes?”
Mr. Pozela said he was not yet authorized to disclose any findings of the investigation.
The chorus of condemnation and outrage from across the European Union swelled overnight as leaders began discussing possible penalties they could direct at Belarus for its forcing down of a civilian passenger jet.
However, they are somewhat limited in the actions at their disposal, because there are already E.U. sanctions against Aleksandr G. Lukashenko, the brutal and erratic leader of Belarus who has clung to power despite huge protests against his government last year, and dozens of his immediate associates.
In a summit scheduled to take place Monday evening, European leaders are expected to discuss adding aviation-related sanctions.
The options may include designating Belarusian airspace unsafe for E.U. carriers; blocking flights from Belarus from landing in E.U. airports, and sanctions against the national flag carrier, Belavia.
E.U. leaders also called for an investigation into the circumstances of the incident by the International Commercial Aviation Organization.
Prime Minister Kyriakos Mitsotakis of Greece, where the flight originated, said it was critical the European Union take specific action, especially in the context of the bloc’s frequent paralysis over foreign-affairs issues including a recent failure to agree on a statement regarding the Middle East conflict.
“Our inability to reach a consensus on recent events in Israel and Gaza — where as a union we failed to present a unified stance — must not be repeated,” Mr. Mitsotakis told the Financial Times. “The forcible grounding of a commercial passenger aircraft in order to illegally detain a political opponent and journalist is utterly reprehensible and an unacceptable act of aggression that cannot be allowed to stand.”
Ursula von der Leyen, the president of the European Commission, the bloc’s executive arm, also promised action at the leaders’ summit.
“The outrageous and illegal behavior of the regime in Belarus will have consequences,” she said in a tweet Sunday evening, adding that there must be sanctions for those “responsible for the #Ryanair hijacking.”