Liu Yaping, a permanent U.S. resident, was detained March 8 and has been accused of fraud and tax evasion, though police presented no evidence, said Jerome Cohen, a New York University law professor.
Liu, 48, was healthy when he was detained, but has received emergency medical treatment at least three times and may have been tortured, Cohen said in Beijing.
"This is a man who has an aneurysm on an artery that leads to the brain," he said. "This guy could die at any time."
People who have seen Liu report that his in "bad shape," unable to keep food down and very agitated over authorities' refusal to let him see family or a lawyer, Cohen said. He did not specify who had seen Liu.
Cohen said Liu could be treated even more harshly to punish his family for publicizing his case. "There's a risk that they might really beat the hell out of him," he said.
China's Foreign Ministry had no immediate response Tuesday to questions about Liu.
Liu was arrested in the city of Hohhot in China's Inner Mongolia region, where he had started a Web site design company. Liu has permanent U.S. resident status. His wife and son, who live in Connecticut, are American citizens.
Liu's case adds to a wave of detentions of U.S.-linked academics, writers and business people in China that prompted the U.S. State Department to issue a warning to travelers to China.
Cohen is also petitioning to see another of the recent detainees: Gao Zhan, 39, a university researcher and U.S. permanent resident who was detained Feb. 11 and has been charged with espionage.
Cohen said Liu's detention might be revenge for a business or personal dispute and that the delay in resolving his case could be the result of power struggles within the security forces.
Liu was detained on suspicion of commercial fraud, Cohen said. Three weeks later, police accused him of tax evasion, and later added the charge of filing false documents in connection with setting up a foreign-owned company, Cohen said.
Cohen said Chinese trade officials have told him they had never heard of such cases resulting in criminal charges.
"This only sustains the notion that this man was locked up for reasons unrelated to the charges," Cohen said.
Hohhot police have rebuffed repeated attempts by Chinese lawyers to visit Liu, despite legal guarantees of such access, Cohen said.
After a visit by prominent Beijing lawyers that apparently unsettled Hohhot police, local authorities announced that Liu would be denied access to counsel because his case involved "state secrets," Cohen said.
A spokesman for the U.S. Embassy in Beijing said diplomats have conveyed questions from Liu's wife about his condition to Chinese authorities and provided her with the responses.
U.S. citizens detained in China are guaranteed access to American diplomats b treaty, but such access does not extend to permanent residents such as Liu.
The embassy has appealed to Chinese authorities to ensure Liu has access to lawyers, said the spokesman, who declined to provide further details, citing privacy rules.
Police in Hohhot said they had no information on Liu's case. A prosecutor in the city said, "Everything before public trial is confidential."
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