Whether that security alert was a fabrication by the Belarus authorities is now at the heart of an incident that has sparked widespread international condemnation and raised serious questions about safety in the skies. Some governments have described the incident as a state-sanctioned hijacking.
One of the passengers onboard the Ryanair flight from Athens to Vilnius was Belarus opposition activist Raman Pratasevich, who is wanted on a variety of charges. For him, the diversion was much more than an inconvenience. As soon as the plane landed, he was arrested, according to the Belarus Interior Ministry.
The President of Belarus, Alexander Lukashenko, has been fending off opposition protests since claiming victory last year after a hotly-disputed election widely condemned by the international community.
Pratasevich is one of dozens of journalists and activists campaigning in exile against Lukashenko's 26-year rule. He is the founder of the Telegram channel Nexta, which helped mobilize anti-Lukashenko protests, and was charged last year with "organizing mass riots and group actions that grossly violate public order." He is on a government-wanted list for terrorism.
Just why the plane suddenly changed course depends on whose account one believes. Ryanair says that its crew was "notified by Belarus ATC [air traffic control] of a potential security threat on board and were instructed to divert to the nearest airport, Minsk."
That's not how the Belarus authorities characterized the incident. The Deputy Commander of Air Defense Forces, Major-General Andrey Gurtsevich, claimed that after the Ryanair crew were told of a "possible bomb on board," it was the captain who "made a decision to land at the reserve airfield (Minsk-2)."
Gurtsevich said a Belarus Air Force MiG29 jet was dispatched to monitor the flight and "assist" if necessary.
The Belarus version of events has been met with widespread disbelief and condemnation among the international community, despite an elaborate show of fire trucks when the plane landed, as well as extensive baggage checks. Nothing untoward was found, according to Ryanair.
One reason for skepticism toward the Belarusian authorities' version: When it changed course, the Ryanair Boeing 737 -- with 171 people on board - was much closer to its destination than it was to Minsk. Had there been a bomb on board, to prolong the flight would have been a perverse decision.
Europe and the US dismiss the Belarus government's explanation
European Commission President Ursula von der Leyen tweeted that it was "utterly unacceptable to force @Ryanair flight from Athens to Vilnius to land in Minsk."
"The outrageous and illegal behaviour of the regime in Belarus will have consequences. Those responsible for the #Ryanair hijacking must be sanctioned. Journalist Roman Protasevich must be released immediately," von der Leyen said in a later tweet.
Josep Borrell, the European Union's top diplomat, said the forced landing was "totally inadmissible," while France's Transport Minister Jean-Baptiste Djebbari was blunter still, tweeting that "hijacking a plane is unacceptable."
British Foreign Secretary Dominic Raab said that "this outlandish action by Lukashenko will have serious implications."
US also diplomats also weighed in, with US Ambassador to Belarus Julie Fisher tweeting, "Lukashenka and his regime today showed again its contempt for international community and its citizens.
"Faking a bomb threat and sending MiG-29s to force @RyanAir to Minsk in order to arrest a @Nexta journalist on politically motivated charges is dangerous and abhorrent."
A senior Republican in Congress, Rep. Michael McCaul said on Twitter. "To force an Irish aircraft with nearly 200 innocent civilians to land in order to make that arrest is an egregious affront to democratic societies around the world."
The flight was traveling from one EU city to another, briefly passing through Belarus' airspace, and its diversion has serious implications for internationally accepted rules of aviation travel.
The challenge for the European Union and its allies is to come up urgently with actions to reinforce their strong words, as German Foreign Minister Heiko Maas acknowledged when he tweeted "This cannot remain without clear consequences on the part of the EU."
But Lukashenko has previously shrugged off EU sanctions aimed at him and his inner circle. It is - after all - not a group that's deeply dependent on the outside world.
Deeper sanctions would hurt the people of Belarus as much as the regime. In any case, Belarus is not economically beholden to western Europe. The EU accounts for less than 20% of Belarus' trade; Russia accounts for 49%.
The EU had already suspended moves towards closer economic relations with Belarus before last year's widely criticized elections.
It's unclear whether the European Union will take up the incident with the International Civil Aviation Organization, the body entrusted with regulating the world's skies. The ICAO told CNN Sunday that "for the time being this represents a bilateral matter between the specific countries involved. Any investigations would need to be undertaken by suitable national authorities," who could submit a complaint to ICAO's Council or Assembly.
ICAO later said it was "strongly concerned by the apparent forced landing of a Ryanair flight and its passengers, which could be in contravention of the Chicago Convention." The convention governs rules of airspace, aircraft registration and safety.
At dusk Sunday, flight 4978 finally left Minsk after seven hours on the ground -- but minus one very significant passenger. Other opposition figures now fear for Pratasevich's safety.
Sviatlana Tsikhanouskaya, who ran against Lukashenko last year and is now in exile in Lithuania, told CNN Sunday: "We really don't what's happening to him (Pratasevich) right now and we know how cruel [Belarus security service] can be."
Tsikhanouskaya said she had been on the same flight from Athens to Vilnius last week. "The escalation of repressions in Belarus, and this situation that's happened with [the] flight is a result of impunity," she told CNN.
"Democratic countries should put much more pressure on this regime on Lukashenko personally to make him understand that."
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