BEIJING — After two decades of working as a successful engineer in the United States, Hu Zhicheng decided to return to China in 2004 and apply his rich experience to designing catalytic converters for the nation’s booming automotive industry.

“I saw how polluted the air was here, and thought I could make a difference,” said Mr. Hu, a naturalized American citizen who has a doctorate in engineering.

Now it seems he cannot leave.

The last three times he tried to board an airplane and return to his family in Los Angeles, Mr. Hu, 49, was turned away by Chinese border agents who claimed that he was a wanted man.

The problem is, he cannot find out exactly who wants him and why.

Mr. Hu, an inventor trained at the Massachusetts Institute of Technology with 48 patents and a number of prestigious science awards to his name, was jailed for a year and a half starting in 2008 after a former business associate accused him of commercial theft. The charges were so spurious that prosecutors withdrew the case — a rare gesture in China’s top-down legal system.