On May 23rd, Belarusian authorities scrambled a fighter jet and flagged what turned out to be a false bomb alert to force a Ryanair plane to land.

When the civilian plane landed, authorities arrested an opposition figure who was on board, drawing condemnation from Europe and the United States.

Roman Pratasevich was arrested after the flight in which he was traveling was diverted to Belarus, allegedly after a bomb threat, in what the opposition and Western officials denounced as a hijacking operation by the government.

Four others also didn’t complete the trip to Vilnius.

Passengers were taken off the plane in Minsk.

After the plane arrived in Vilnius, Defense Minister Arvydas Anusauskas said Pratasevich’s girlfriend and four other people did not reboard.

“We will find out who are the other four that did not travel with the rest. Lithuania has launched an investigation to find out what really happened on that plane,” he said on Facebook.

Flight tracker sites indicated the plane was about 10 kilometers (six miles) from the Lithuanian border when it was diverted. There were conflicting accounts of the move.

The press service of Belarusian President Alexander Lukashenko said the president himself ordered that a MiG-29 fighter jet accompany the airliner after he was informed of the bomb threat. Deputy air force commander Andrei Gurtsevich said the plane’s crew made the decision to land in Minsk.

Ryanair said in a statement that Belarusian air traffic control instructed the plane to divert to the capital.

Lithuanian President Gitanas Nauseda called the incident a “state-sponsored terror act.” He said that the European Council would discuss the case Monday and that he would propose banning Belarusian planes from European Union airports and “serious sanctions” against Lukashenko’s government.

“Belarusian airspace is completely unsafe for any commercial flight, and it should be deemed this not only by the EU but by the international community. Because now, this instrument could be used for any plane crossing Belarusian airspace,” said Lithuania’s foreign minister, Gabrielius Landsbergis.

Pratasevich is a co-founder of the Telegram messaging app’s Nexta channel, which Belarus last year declared as extremist after it was used to help organize major protests against Lukashenko. The channel is operated from Poland.

Opposition leader Sviatlana Tsikhanouskaya called on the International Civil Aviation Organization to begin an investigation.

“It is absolutely obvious that this is an operation by the special services to hijack an aircraft in order to detain activist and blogger Raman Pratasevich,” she said in a statement. “Not a single person who flies over Belarus can be sure of his safety.”

NATO Secretary-General Jens Stoltenberg said in a tweet that the incident was serious and dangerous and required an international investigation.

Other EU leaders and foreign ministers condemned the incident.

EU High Representative for Foreign and Security Policy Josep Borrell called for an international investigation into the incident with the Ryanair landing in Minsk.

The head of the US Senate Foreign Affairs Committee Bob Menendez, together with colleagues from the Czech Republic, Latvia, Germany, Lithuania, Ireland, Poland and the UK, issued a statement regarding the incident with an emergency landing of the Ryanair plane in Minsk and called for a ban on all flights over Belarus.

“We call on the International Civil Aviation Organization (ICAO) to immediately investigate this apparent violation of free transit between states and threats to aviation,” said a statement released by Menendez’s press office.

The UN’s International Civil Aviation Organization (ICAO) said the incident may have contravened a core aviation treaty, part of the international order created after World War Two.

“ICAO is strongly concerned by the apparent forced landing of a Ryanair flight and its passengers, which could be in contravention of the Chicago Convention,” it said.

“We look forward to more information being officially confirmed by the countries and operators concerned.”

Airlines joined a flurry of government protests.

“We strongly condemn any interference or requirement for landing of civil aviation operations that is inconsistent with the rules of international law,” said the International Air Transport Association.

“A full investigation by competent international authorities is needed.”