MOSCOW (NYTIMES) - She was one small step away from finishing law school. Post-university life was dawning, seemingly full of promise. Friends planned parties and summer vacations.

But Sofia Sapega, 23, instead now sits in a cell in Belarus, showing how a life can be abruptly upended by the harsh recent pivot in Belarusian and Russian politics, even for a young woman who was relatively apolitical, according to acquaintances.

Ms Sapega was arrested last month after the Belarusian authorities forced down a Ryanair flight in the country’s airspace as she was travelling between Greece and Lithuania.

The Belarusian authorities have not formally disclosed their accusations, but, informally, the reason for her arrest seems clear enough: Her crime was having a boyfriend, Roman Protasevich, who was prominent in the post-Soviet country's opposition. The two were detained together while returning from a vacation.

"All the students in our group were shocked and upset," said Ms Katsiaryna Shafranovich, a classmate of Ms Sapega's at the European Humanities University in Lithuania. "Some students don't pass exams and don't graduate. But this is different. This is not fair." The two students shared a thesis adviser throughout the five-year law school course at the university, which was forced to relocate to Vilnius, Lithuania, after Belarus authorities closed the school in 2004.

"She's a girl with her dreams, her desire to finish university, to have her life," Ms Shafranovich said of her classmate. "Now it's all ruined."

While the Belarusian authorities have jailed Ms Sapega, the real target of the security services, according to opposition leaders, was Mr Protasevich, 26, a journalist and activist.

If Mr Protasevich is convicted on certain terrorism charges, he could be executed. The Belarusian authorities have a track record of also targeting relatives and loved ones of prominent dissidents. It's unclear how long Ms Sapega might remain incarcerated.

The capriciousness of justice and its human cost in Ms Sapega's case came into focus this week when her mother provided the BBC with letters she had written from detention in Minsk, the Belarusian capital. She wrote that she felt "tormented" by what she was missing in life.

"I should have defended my dissertation today and gone to a restaurant with Roma in the evening to drink champagne," she wrote. Instead, she had biscuits and tea alone in her cell, the BBC reported. "It's so sad to think that so much else will happen in life that I will miss," she wrote. "I didn't want to write about this, but I have nobody to share things with and I couldn't stop myself. Forgive me."

The couple had been flying from Athens to Vilnius on May 23. In the final minutes of its journey, the plane passed over Belarusian airspace. Dispatchers directed the pilot to land, saying the plane faced a security threat.

Western governments say the Belarusian government used the ruse of a bomb threat to force down the flight. European nations now divert flights around Belarus.

Street protests broke out in Belarus last summer after President Alexander Lukashenko claimed victory in an election his opponents said was rigged, and a police crackdown ensued. More than 30,000 people have been arrested and hundreds remain in prison.

Mr Protasevich is a co-founder and a former editor of the NEXTA channel on the messaging app Telegram, which has become a popular conduit for Mr Lukashenko's foes to share information and organize demonstrations.

As the Ryanair plane descended toward Minsk, other passengers said, Mr Protasevich pleaded with the flight attendants not to land, saying he might be killed as a prominent voice in the opposition.

Ms Sapega, in contrast, was not publicly an unusually political person, according to Ms Shafranovich, her classmate. The Belarusian police have released a video of Ms Sapega confessing to editing an online opposition publication called the Black Book Of Belarus which released personal information about security officials. But such videotaped confessions, apparently made under duress, are commonly disregarded outside Belarus.

Although raised from a young age in Belarus, Ms Sapega is a Russian citizen. But hopes that Russia might intervene waned after the countries' presidents, Lukashenko and Vladimir Putin, met this month and discussed the case.

Russia itself has intensified pressure on its domestic opposition this year. Mr Alexei Navalny, a leading critic of Putin, survived an assassination attempt in August and when he recovered was imprisoned. The Kremlin denied any role in the poisoning.

"The investigation of both persons will take place in Belarus," Mr Lukashenko said of the couple. Russian officials did not mind, he said. "In my opinion, they are not offended."

European Humanities University spokesman Maksimas Milta said Ms Sapega will be welcomed back to defend the dissertation she already submitted if she is freed. The university has refunded tuition from this year to help with legal expenses. "Because of this bizarre and unacceptable situation, she was placed on academic leave," Ms Milta said. "She remains a student. As soon as possible she will be allowed to defend her thesis."

Since the day the plane landed in Minsk, Ms Sapega's mother Anna Dudich has been struggling to find information about why her daughter was detained.

"The last time my daughter and I spoke on the phone was on the evening before their departure from Athens," Ms Dudich told Deutsche Welle. "She said they would fly the next day and had a good vacation."

Back at the university, Ms Shafranovich has already defended her law dissertation and is looking forward to graduation next month.

Ms Shafranovich said she had written to her friend in detention but was unsure the letter passed prison censors, though she had tried to keep it bland. "I just asked if she had a possibility to read and what book she would like," she said. "I only wanted to make her mind forgetful, even for a moment, about where she is."