Chinese rights lawyer Chen Jiangang, who helped to expose the torture of his client and fellow lawyer Xie Yang, was in the custody of China's state security police on Friday.
Chen went 'missing' with his wife Zou Shaomei and the couple's two sons during a family trip to the southwestern province of Yunnan after making a video saying he was concerned that he might be detained.
An officer who answered the phone at the municipal state security police bureau in Yunnan's Jinghong city appeared to confirm the agency's involvement.
"Don't you worry about them, they're just fine," the officer said when contacted by RFA on Thursday. "OK, let's leave it there."
Chen and his family were detained on Wednesday afternoon, activists close to Chen said via social media.
His family was released Thursday morning and permitted to take a flight back to Beijing, but Chen was forced drive the 3,200-kilometer (2,000-mile) journey home with three police escorts, Agence France-Presse reported.
Sources said Zou and the children remain under police surveillance at their home, while Chen's current whereabouts are unknown.
A fellow rights lawyer who asked to remain anonymous said Chen may have been hoping to help his family flee China by crossing the border into northern Myanmar from Yunnan.
"They were on a trip, but the possibility that he wanted to take his wife and kids out of the country can't be ruled out," the lawyer said. "Our biggest concerns are for his family, because I think it unlikely that Chen himself will want to leave."
"A lot of people are sending their wives and kids overseas now, because they all have the same fears ... and it's very common for the Chinese authorities to retaliate against loved ones [of dissidents]," he said.
"We see a lot of cases where they target the relatives for speaking out against the injustice [faced by the dissident or activist]," he said.
The family was detained alongside rights activist Zhang Baocheng and his wife, who also remain incommunicado, sources told RFA.
Repeated calls to Chen and Zou's cell phones rang unanswered on Wednesday, while an employee who answered the phone at the Mengyang police station in Yunnan where the six people were detained declined to comment.
Rights lawyer Ge Yongxi said he had received a brief smartphone chat message, including a location notification, from Zou, indicating that the family had been detained.
"She sent me a WeChat message [on Wednesday] ... but I can't confirm any other details because I haven't spoken directly with Chen Jiangang's wife," Ge said.
"But Chen Jiangang should be at liberty, because he was taking a trip within China's borders," he said.
UN weighs in
The United Nations human rights agency on Friday denounced China’s ongoing detention and harassment of human rights lawyers and activists, including Chen.
"Prior to being reportedly taken by police last Wednesday, Chen had, in a video message, expressed concerns that he too may ‘lose his freedom’ and that he may be coerced into self-incrimination," U.N. human rights spokeswoman Ravina Shamdasani told reporters in Geneva.
"We are dismayed by this continuing pattern of harassment of lawyers, through continued detention, without full due process guarantees and with alleged exposure to ill-treatment and coercion into self-incrimination," she said.
The U.N. human rights commission said the vast majority of rights lawyers detained or placed under surveillance or travel bans by China since July 2015 were defending the basic rights of Chinese citizens, mostly economic, social and cultural rights.
"We urge the Chinese government to abide by its international human rights obligations, to ensure due process and fair trials, and to release without delay those being held for exercising their fundamental human rights or for defending the exercise of such rights by others," it said in a statement on Friday.
The reports of Xie Yang's torture in police detention were slammed by China's state-run media as "fake news," but Chen stood by them, saying they were "totally accurate."
He has since been prevented from meeting with Xie, and police say he has been replaced with a government-appointed lawyer.
Xie has lodged a formal complaint saying that he was subjected to confinement in a "hanging chair" made of plastic chairs stacked high above the ground for hours at a time, so that his legs swelled up and he was in excruciating pain.
He was also deprived of sleep and repeatedly beaten, humiliated, and taunted with death threats against his family, according to his lawyers' notes.
Reported by Xin Lin for RFA's Mandarin Service, and by Ng Yik-tung and Wen Yuqing for the Cantonese Service. Translated and edited by Luisetta Mudie.