She was a gifted agricultural scientist educated at prestigious universities in Shanghai and Tokyo. She said she wanted to help farmers in poor areas, like her hometown in Xinjiang, in western China. But because of her uncle’s activism for China’s oppressed Muslim Uyghurs, her family and friends said, the Chinese state made her a security target.

At first they took away her father. Then they pressed her to return home from Japan. Last year, at age 30, Mihriay Erkin, the scientist, died in Xinjiang, under mysterious circumstances.

The government confirmed Ms. Erkin’s death but attributed it to an illness. Her uncle, Abduweli Ayup, the activist, believes she died in state custody.

Mr. Ayup says his niece was only the latest in his family to come under pressure from the authorities. His two siblings had already been detained and imprisoned. All three were targeted in retaliation for his efforts to expose the plight of the Uyghurs, he said.

“People are not only suffering there, they are not only being indoctrinated there, not only being tortured, they are actually dying,” said Mr. Ayup, who now lives in Norway. “And the Chinese government is using this death, using these threats to make us silent, to make us lose our hope.”

As Beijing has intensified its repression in Xinjiang in recent years, more Uyghurs living overseas have felt compelled to speak out about mass internment camps and other abuses against their families back home. Their testimonies have added to a growing body of evidence of China’s crackdown, which some have called a genocide, prompting foreign governments to impose sanctions.

Credit…Abduweli Ayup

Now the Chinese authorities are pushing back against overseas Uyghurs by targeting their relatives.

The Communist Party has long treated the relatives of dissidents as guilty by association and used them to pressure and punish outspoken family members. With the courts under the control of the authorities, there is little recourse to challenge such prosecutions. Liu Xia, the wife of Chinese activist Liu Xiaobo, spent nearly eight years under house arrest after he was awarded the Nobel Peace Prize in 2010. Her younger brother, Liu Hui, served two years in prison for a fraud conviction she called retaliation.

But with the Uyghurs, the authorities seem to be applying this tactic with unusual, and increasing severity, placing some Uyghur activists’ relatives in prison for decades, or longer.

Dolkun Isa, the German-based president of the World Uyghur Congress, a Uyghur rights group, said he believes his older brother is in detention. He learned in late May that his younger brother, Hushtar, had been sentenced to life in prison. “It was connected to my activism, surely,” Mr. Isa said.

Radio Free Asia, a United States-funded broadcaster, says that more than 50 relatives of journalists on staff have been detained in Xinjiang, with some held in detention camps and others sentenced to prison. The journalists all work for the broadcaster’s Uyghur language service, which has in the past several years stood out for its reporting on the crackdown, exposing the existence of camps and publishing the first accounts of deaths and forced sterilizations.

The sister of Rushan Abbas, a Uyghur American activist, was sentenced in December to 20 years in prison for terrorism. The sister, Gulshan Abbas, and her aunt had been detained in 2018, days after Rushan Abbas spoke at an event in Washington denouncing the crackdown and widespread detention in Xinjiang.

“As retaliation against me because I made that public speech, as a tool to silence me, they abducted my sister,” Ms. Abbas said. “They have my sister as a hostage right now.”

At Beijing’s request, some countries have also sent more than 300 Uyghurs back to China since 2010, according to a study by the Oxus Society for Central Asian Affairs, and the Uyghur Human Rights Project, nonprofits based in Washington, D.C. One Uyghur now fighting extradition is Idris Hasan, whom activists say has been detained in Morocco.

In the case of Ms. Erkin, the scientist, her uncle first drew the attention of the authorities in Xinjiang for trying to expand the use of the Uyghur language. The government regarded even the most moderate expression of ethnic identity as a threat and Mr. Ayup was arrested in 2013 and spent 15 months in prison. After he was released, he fled abroad, but his experience emboldened him to continue campaigning.

Back home, Mr. Ayup’s brother, Erkin Ayup, a local Communist Party official, knew that his own situation was precarious. In 2016, he told his daughter that a crackdown was unfolding, and he feared he could be caught up in it, according to Asami Nuru, a friend of Ms. Erkin’s in Tokyo.

The father and daughter devised a simple system to let Ms. Erkin know he was safe: he would send her a smiley face sticker on WeChat every morning.

“One day, he didn’t send the sticker,” Ms. Nuru said. “She called her mother and she learned her father was in a camp. She was very upset, and from then on she would cry every day.”

Mr. Ayup believes the authorities took his brother into custody in mid-2017.

In the years that followed, Ms. Erkin’s anxiety over her father’s situation bore down on her, and she even lost weight, Ms. Nuru said. She began to receive adamant messages from her mother, likely at the behest of the authorities, telling her to stop her uncle’s activism or return home.

Her family and friends say her decision to return to China in June 2019 was sudden. She left her suitcases in the house where she lived.

Ms. Erkin called Ms. Nuru from the airport and told her that she wanted to try to find her father, even though she knew he was still in detention. Ms. Nuru said she tried to persuade her against the idea.

“She told me, ‘I want to try to find my father, even if it means I might die,’” Ms. Nuru said.

Mr. Ayup says he believes that the authorities arrested Ms. Erkin in February 2020 to punish him after he helped international news outlets report on a leaked government document outlining how Uyghurs were tracked and chosen for detention.

The circumstances of Ms. Erkin’s death remain unclear.

Her death was first reported by Radio Free Asia, which cited a national security officer from Ms. Erkin’s hometown as saying she had died while in a detention center in the southern city of Kashgar. Mr. Ayup said he believed it was the same place where he himself had been beaten and sexually abused six years earlier.

Ms. Erkin’s family was given her body, Mr. Ayup said, but were told by security officials to not have guests at her funeral and to tell others she died at home.

In a statement to The New York Times, the Xinjiang government said that Ms. Erkin had returned from overseas in June 2019 to receive medical treatment. On Dec. 19, she died at a hospital in Kashgar of organ failure caused by severe anemia, according to the statement.

From the time she went to the hospital until her death, she had always been looked after by her uncle and younger brother, the government wrote.

Before she returned to China, Ms. Erkin seemed to be aware that her return could end tragically.

“We all leave alone, the only things that can accompany us are the Love of Allah and our smile,” she wrote in text messages to Mr. Ayup when he tried to dissuade her from going home.

“I am very scared,” she admitted. “I hope I would be killed with a single bullet.”