U.S. intelligence agencies interrogating the brother of a disgraced Communist official believe he is the most valuable Chinese defector to flee to America, according to two people familiar with some of the intelligence he has provided.
The defector, Ling Wancheng, is the brother of Ling Jihua, the former chief of staff to President Hu Jintao who was formally detained on suspicion of "serious violations" of Communist party rules in December 2014.
The secrets Mr Ling has revealed to US investigators include details on Chinese procedures for launching nuclear weapons, the personal lives of China's leaders, and arrangements for their security and for the protection of the Zhongnanhai leadership compound in central Beijing, according to one senior retired diplomat and a former leading western intelligence official who received briefings in Washington.
In a sign of how badly it wants to get him back, the Chinese government has sent several teams of security officials and agents to the US on official and covert missions to try to secure Mr Ling's return.
Last August the Obama administration issued a warning to Beijing after discovering that Chinese spies in the US were trying to track Mr Ling down and repatriate him.
In November an official delegation from the Chinese Ministry of Public Security travelled to the US to present accusations against Mr Ling to the Sacramento Federal Prosecutor.
The Chinese delegation initially alleged Mr Ling had laundered enormous sums of money through the US but it was unable to provide enough evidence to satisfy US prosecutors.
During a visit to Washington in early September, Meng Jianzhu, China's top security official, also pressed the Obama administration to return Mr Ling to China to face prosecution in connection with his brother's alleged crimes.
The White House, the Central Intelligence Agency and the Federal Bureau of Investigation could not immediately be reached for comment.
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In his position as director of the general office of the Communist party of China between 2007 and 2012, Ling Jihua was the top aide to President Hu Jintao and was responsible for categorizing and archiving all of the party's most secret and sensitive information.
Hong Kong-based media reports alleged late last year that Ling Jihua had stolen thousands of classified documents and handed them over to his brother Wancheng, who transferred them to the mansion he owns in California, near Sacramento.
Ling Jihua last appeared in public in October 2014 and in July last year Chinese state media reported he had been expelled by the party and charged with several crimes and violations of party discipline, including corruption, adultery and stealing state secrets.
The official government announcement at the time said he had "obtained a great deal of the party and state's core secrets in violation of laws and discipline", "accepted huge bribes" and "committed adultery with a number of women and traded his power for sex".
These charges marked the culmination of a spectacular downfall that began in early 2012 when Ling Jihua's 23-year-old son was killed in a car crash while driving a Ferrari in Beijing city centre with two young women, one naked.
Despite a media blackout and government attempts to cover it up, the event was widely reported by international news organisations and Ling Jihua was moved to a less sensitive government position later that year.
Until now, the most valuable Chinese defector to the US was widely believed to be Yu Qiangsheng, spymaster from China's Ministry of State Security and son of senior party members, who fled to America in 1985.
His defection led to the arrest and conviction of CIA analyst Larry Wu-Tai Chin on charges of spying for China. Mr Chin was found dead in 1986 in his prison cell from apparent suicide just days before he was to be sentenced.
Yu Qiangsheng was later assassinated by Chinese agents, according to Chinese officials familiar with the matter. Mr Yu's younger brother, Yu Zhengsheng, is now a member of the seven-man Politburo Standing Committee, the highest political body in China.