Chinese authorities have begun to round up relatives and associates of blind activist Chen Guangcheng, who fled from house arrest last week, reports say.
Several people involved in Mr Chen's escape have been detained or have disappeared in recent days, and fellow activist Hu Jia was questioned.
Mr Chen is believed to be sheltering at the US embassy in Beijing.
The US and international rights groups have frequently expressed alarm at the treatment of Mr Chen and his family.
US Secretary of State Hillary Clinton, who has demanded his release in the past, is due in China this week for a previously arranged meeting which is now likely to be overshadowed by Mr Chen's case.
The US government has not so far commented publicly on the whereabouts of Mr Chen.
Analysts say the issue will be highly sensitive for both sides, and will not be easy to resolve.
If Mr Chen is in the embassy, his case will raise memories of an incident in 1989 when another prominent activist, Fang Lizhi, fled to the US mission in Beijing.
He remained there for more than a year while the two sides attempted to broker a deal.
Mr Chen was placed under house arrest in 2010 after spending more than four years in jail for disrupting traffic and damaging property.
He had exposed how local authorities in Linyi, Shandong province, forced thousands of women to have abortions or be sterilised as part of China's one-child policy.
His colleagues said last Sunday's escape had taken months to plan, and was carried out with the help of a network of friends and activists.
He scaled the wall that the authorities had built around his house, and was driven hundreds of miles to Beijing, where activists say he stayed in safe-houses before fleeing to the embassy.
His wife and six-year-old daughter remain under house arrest, but several of his family members have been detained and others are being sought by the authorities.
One of Mr Chen's friends, He Peirong - who wrote on her microblog that she had driven him to Beijing - is believed to have been detained in the city of Nanjing.
"I was actually talking to her and the last words she said were 'the PSB [Public Security Bureau] has arrived,'" said Bob Fu, of the US-based ChinaAid pressure group.
Her microblog was later deleted, and all searches on popular microblogging sites for Mr Chen's name and other related terms were being blocked by the censors.
On Saturday, the authorities detained Hu Jia, who had earlier told the BBC how he had met Mr Chen since his escape.
Mr Hu was reportedly allowed home on Sunday.
The fate of other associates of Mr Chen also remains unclear, with reports claiming several have disappeared.
The treatment of Mr Chen and his family by local authorities has long been controversial.
Amnesty International regards him as a "prisoner of conscience" and has called on the authorities to end the "shameful saga" of his detention.