Xue Feng, 50, was deported immediately after his release and has returned to his family in the US city of Houston, the San Francisco-based Dui Hua foundation said on Friday.

"Dui Hua is delighted that Dr. Xue has finally been reunited with his family in America after a terrible ordeal," the group's executive director John Kamm said in a release. The group advocates clemency and better treatment for prisoners in China.

Xue was detained in November 2007 and sentenced in 2010 on charges of having illegally collected data on China's oil industry. His arrest came in connection with the purchase of a database containing industrial information, which both Xue and the US energy and engineering consulting firm he worked for, IHS, said they believed was a commercially available product.

Dui Hua says the database was classified as a state secret only after Xue bought it.

The conviction against Xue was upheld in February 2011, but was reduced by 10 months for good behavior in November 2011, according to Dui Hua. He served his sentence in Beijing No. 2 Prison.

Rights activists say China's state secret laws are often abused to silence government critics.

Xue's case raises questions about doing business in China and whether normal business dealings elsewhere in the world could contravene the country's state security legislation, which allows considerable leeway in its application.

His release means there are no more US citizens serving sentences in China on state secret related charges, Dui Hua said.

tj/lw (AP, AFP)