Beijing (AFP) - When American journalist Gulchehra Hoja and her colleagues first began exposing a secretive network of reeducation centres in China's far west, they never imagined that their families might one day end up in one.
But when calls to relatives went unanswered, they realised that something had gone terribly wrong in Xinjiang, home to China's Uighur Muslim minority.
"It was like a bomb going off in my head," said Hoja, a reporter at the US government-funded Radio Free Asia (RFA) news service, describing how she felt when she learned that as many as 20 of her relatives had disappeared.
She and her colleagues fear Chinese authorities snatched their family members as punishment for their reports on the mass detention of Uighurs in "political reeducation centres" across Xinjiang.
The authorities are "using my brother as a kind of hostage to pressure me to quit", said Mamatjan Juma, an RFA reporter who believes two of his brothers have been detained since May.
In recent months, Chinese authorities have cracked down hard on Xinjiang's 10 million Uighurs, placing draconian restrictions on their daily lives in the name of combating terror.
Hoja is one of six Uighur reporters whose families are believed to have been detained by Chinese authorities, according to RFA spokesman Rohit Mahajan, who said the company has discussed the cases with the US State Department and members of Congress.
"We urge the Chinese government to provide information about the health of the missing relatives and to return them safely to their homes post-haste," Libby Liu, RFA's president, said in a statement.
US embassy Beijing spokeswoman Jinnie Lee told AFP Washington is "concerned" by the detentions and "deeply troubled by reports of an ongoing and deepening crackdown on Uighurs and other Muslims in China".
The Chinese foreign ministry declined to comment.
Some of the reporters have waited almost a year for movement on the cases, but decided to make their stories public after seeing no progress.
- 'Political reeducation' -
RFA's Uighur service was launched in December 1998 to broadcast Uighur language news into China, part of the company's mandate to provide news to Asian countries "whose governments prohibit access to a free press".
The journalists began reporting about the mass disappearances late last spring.
People of all ages were being swept up by the police, often for seemingly trivial offences related to religion, from sending Ramadan greetings to friends on social media to growing a "strange" beard.
By following official social media accounts and calling local police, the reporters learned that tens of thousands were being sent to detention centres for months at a time and subjected to intense political indoctrination.
Chinese authorities have long linked their crackdown on Uighur Muslims to international counter-terrorism efforts, arguing that separatists are bent on joining foreign extremists like Al-Qaeda.
Uighurs have been tied to mass stabbings and bombings that left dozens dead in recent years across the country. Riots and clashes with the government killed hundreds more.
Chinese authorities have denied the detention centres exist.
But regulations aimed at curbing religious extremism adopted by Xinjiang's government last March clearly call for authorities to step up the use of political reeducation.
An AFP review of state media reports and government documents verified the existence of at least 30 such centres and almost 4,000 cases of people being sent to them.
Shortly after RFA began posting its first reports on the centres, information about them began to disappear from Chinese websites.
- 'Voice of the voiceless' -
Hoja started a career in local television at the age of 11 and had become a popular television host in Xinjiang.
But a radio broadcast from RFA changed her life: "It was so powerful. I'd never heard that kind of voice. It completely changed me."
She joined RFA in 2001.
When Chinese authorities realised she had fled the country, they began pressuring her parents into making her come back, promising a high-paying job and luxury housing.
During her 17 years at RFA, none of her relatives had been detained -- until last spring.
When she heard the news through friends with relatives living in the US, she frantically tried to contact her family members, but no one answered, she said.
She finally reached an aunt: "She said 'don't call me. Don't call anybody. Just hang up'."
Despite concerns for their families, the reporters vow to continue their work.
"This is the only voice for my voiceless people," Hoja said.
"Other listeners just give their time to listen to the radio. Uighurs give their lives."