A U.S.-based librarian and historian who had been in a Chinese jail for six months on charges of stealing secret documents was released this morning following an unprecedented campaign for his freedom by scholars, U.S. diplomats and politicians.

U.S. experts said the release of Song Yongyi marked the first time a Chinese citizen has been freed after being detained by the Ministry of State Security for crimes related to state secrets.

Song left for the United States today, his wife, Helen Yao, said in a telephone interview from her home in Carlisle, Pa. Yao talked to her husband on Friday for only the third time since Chinese security agents arrested the couple last August. She was released in November.

"He called me from jail. He was in good spirits," she said. "He said he will not stop his research."

The Chinese decision to release Song, a librarian at Dickinson College in Carlisle and a leading expert on China's Cultural Revolution of 1966-76, followed a public campaign for his freedom that culminated in a threat by Sen. Arlen Specter (R-Pa.), to introduce legislation that would have made him an American citizen while in jail.

Song's release also comes as Beijing tries to improve its image in advance of an expected vote in Congress on permanent normal trade status for China. China's ambassador to the United States, Li Zhaoxing, claimed in a statement that Song was released because he had acknowledged his guilt and "voluntarily revealed evidence against illegal activities of others." Song's wife and Jerome Cohen, a legal expert on Chinese affairs, said Song had not admitted doing anything wrong and was forced to sign a blank piece of paper on which the Chinese later wrote a confession.

Participants said the campaign showed that public diplomacy with China is often more successful than the discreet contacts favored by Washington insiders and American "old friends" of the regime.

"We're just ecstatic," said Rep. Matt Salmon (R-Ariz.), who raised Song's case with President Jiang Zemin in a meeting on Jan. 10. "This shows that engagement works."

More than 150 prominent China scholars from several countries signed a petition calling for Song's release and warning China that the case would affect international educational and cultural exchanges. Another petition, posted on the Dickinson Web site, garnered more than 4,000 signatures.

In December, Song's detention caught the attention of Congress, which lobbied Beijing for his release. Salmon led a delegation to China this month and focused on Song's case, and Specter met with Chinese officials in Washington. U.S. Ambassador Joseph Prueher also raised the issue in a recent meeting with Premier Zhu Rongji.

Song was detained on Aug. 5 with Yao as he tried to mail three boxes of documents on the Cultural Revolution to the United States. He was charged on Dec. 24.

Cohen, who helped Dickinson with the case, said Chinese security officials illegally detained Song, tried to plant six pieces of falsified evidence on him and denied him access to a lawyer.

"While the Foreign Ministry was accusing him of committing crimes and blowing this case out of proportion, investigators were pleading with us to provide evidence," Cohen said. "It was ludicrous."

Still, Cohen said, the case represents a breakthrough of sorts for China's fledgling legal system. First, Chinese prosecutors declined to try Song on charges of exporting state secrets, dealing the Ministry of State Security an unprecedented setback. The ministry was then forced to rearrest him, move him from jail to the basement of a building inside the ministry's compound and seek charges for exporting "information"--an even more inexplicit crime.

Second, he said Song's whereabouts were generally known throughout his incarceration.

"Years ago, this never would have happened," Cohen said. "Once the guy was in their clutches, he was gone. The prosecutor would have approved any request of that agency. So there is progress there.

"We're in a new era for China in international relations. In the past, it was like a black hole. Now this doesn't mean the rule of law has come, but it means the ball is in play."

Yao said her husband stressed his determination last night to continue researching the Cultural Revolution--a dark period of Communist China's history in which millions of lives were ruined. Song was jailed for five years during that period. Song co-edited a 20-volume set of documents and was working on another series when he was detained.

"He kept telling me the Cultural Revolution was a world tragedy and he doesn't want it to happen again," Yao said. "He said it would be like letting down the nation."

Song's case sparked an unprecedented outpouring of support from his colleagues in the United States, where his work on the Cultural Revolution has been widely praised. Last year, he completed a paper criticizing Zhou Enlai, China's premier during that period, that is believed to have angered China's propaganda chiefs because Zhou is a revolutionary figure the government has tried to deify.

Special correspondent Cindy Sui contributed to this report.

CAPTION: Song Yongyi, a U.S.-based librarian jailed in China since August, will return to Pennsylvania.