The man, Ge Xun, 53, a naturalized American citizen who moved to the United States from China in 1986, said he was abducted from a street in Beijing this month and was roughly questioned by public security officers at a secret location. During 21 hours of interrogation, Mr Ge said, the agents peppered him with questions about his blogging activity, his membership in an organization that promotes dialogue between Tibetans and Chinese and his role in maintaining a Web site that supports a blind lawyer living under house arrest in China's rural northeast.
But Mr Ge's greatest sin, it appears, was his zealous embrace of Twitter, which has long been blocked in China along with Facebook, YouTube and tens of thousands of other Web sites that the government deems a threat to its hold on power.
In a phone interview on Monday from his home in Fremont, California, Mr Ge described how the agents, infuriated by his assertion that bloggers in the United States were volunteers and not government-sponsored agitators, demanded that he turn over his Twitter password. When he refused, two of them unleashed a torrent of kicks and punches that lasted 30 minutes, he said. "The more they beat me, the less I felt like cooperating," he said.
In the end, Mr Ge and his captors came up with a compromise: he did not reveal his password but logged on to Twitter and allowed them to peek inside his account. "The truth is I have nothing to hide," he said.
Although Mr Ge was released and promptly deported on Feb. 2, the incident highlights the risks faced by foreign passport holders of Chinese origin when ensnared by China's nebulous, omnipotent public security apparatus.
A number of American citizens remain in Chinese prisons on questionable charges, including Xue Feng, a geologist serving eight years for industrial espionage. Another naturalized American, Hu Zhicheng, has been blocked from leaving the country while he battles accusations of commercial espionage lodged by a former business associate. Mr Hu spent a year and a half in jail but was released after Chinese prosecutors acknowledged that the case had no merit.
"Having an American or an Australian passport and having Chinese blood puts you at a disadvantage to those who are white," said Wang Songlian, a researcher at Chinese Human Rights Defenders.
A State Department official declined to comment on Mr Ge's detention but said the plights of Mr Xue and Mr Hu would be raised during the visit to the United States by Vice President Xi Jinping, who arrived in Washington from Beijing on Monday afternoon.
The incident involving Mr Ge was unusual because many native Chinese who hold foreign passports and publicly criticize the Communist Party are denied visas to return home. Mr Ge, who applied and received an emergency visa to attend the funeral of his mother, said he had returned to China numerous times over the years. During a visit in 1997, he said, public security agents briefly and politely questioned him about his lapsed membership in an organization of Chinese students seeking leniency for those arrested during the 1989 crackdown on the Tiananmen Square protests.
"The thing I really care about is basic human rights," said Mr Ge, who studied experimental physics at Texas A&M University and works as a technician for Mercedes-Benz. "I've never called for the overthrow of the government, and I don't advocate violence."
Mr Ge was shocked, he said, when three plainclothes officers accosted him outside the home of Ding Zilin, a retired philosophy professor who has spent two decades seeking justice for those killed during the violent suppression of Tiananmen protesters. Ms Ding's 17-year-old son was killed by a bullet to his heart that June.
The officers not only knew Mr Ge's name but also had a photograph of him that they had downloaded from the Internet. After dragging him into a waiting Honda Accord, they refused to explain what they were after and then confiscated his mobile phone, he said. Once inside a guesthouse called "The Old Cadres Activity Center," he said, they stripped him of his other possessions, including a camera, money and a hand-held recorder.
Then they began questioning him about his activities in the United States and suggested that he had come to China to make trouble on behalf of anti-government forces abroad. He said they simply could not believe that the Web site advocating freedom for the blind lawyer, Chen Guangcheng, was run by volunteers. "That's not possible," Mr Ge quoted one of them as saying. "How can a Web site not belong to an organization, have no leader and not spend any money?"
It was when Mr Ge explained the role of Twitter in spreading word about the site that they demanded his password, he said.
Not long after his pummeling, Mr Ge said, they gave him a sheet of paper and dictated demands that he wrote down with his bruised and wobbly hand. They included a promise not to meet with "sensitive people," not to speak to the news media and not to do anything to harm China's image. He stuck his finger into a jar of red ink and left an imprint signifying his assent.
A few hours later, en route to the airport, he said, he endured another brief beating after refusing to hand over his laptop for one final inspection. Once at the terminal, they returned his camera and recording device, although the contents had been erased. They also handed back the bouquet of flowers he had planned to give to Ms Ding.
As Mr Ge limped away in pain, he said, the lead interrogator, Wang Jie, reminded him that the entire episode was a "national secret." The agent also scribbled down an e-mail address and told him to send a note the next time he came to town. (An e-mail sent to the address seeking comment was not answered on Monday.)
Mr Ge laughed when asked if he might return to China in the near future. "Sure, why not?" he said. "My visa is good for a full year."