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.