Wu Hao, who returned to China in 2004 after 12 years in the states, has been in police custody in the capital since Feb. 22.
His sister, Wu Na, has demanded his release and an explanation for his detention, but police have said his case is "secret."
She said she went to Beijing's Public Security Bureau on Wednesday and was told he had "committed a crime" but officers refused to say anything more.
"They told me there was no way I can see him," Wu said in a telephone interview. "I'm really shocked at this development. This is a place that's administered by law and yet they can't tell me anything."
I'm told the Chinese are making progress on the human rights front, but the evidence is lacking.