The Obama administration has thus far refused to accede to Beijing's demands for Ling Wancheng, and his possible defection could be an intelligence coup at China's expense after it was revealed last month that computer hackers had stolen the personnel files of millions of American government workers and contractors. American officials have said that they are nearly certain the Chinese government carried out the data theft.
Mr. Ling's wealth and his family's status have allowed him to move freely in elite circles in China, and he may be in possession of embarrassing information about current and former officials loyal to Mr. Xi.
Mr. Ling appears to have evaded the Chinese authorities. He is now in the United States, according to several American officials and his next-door neighbor here in the foothills of the Sierra Nevada, where property records show Mr. Ling owns a 7,800-square-foot home, which he bought from a professional basketball player for $2.5 million.
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The Chinese government in recent months has been raising pressure on the Obama administration to return Mr. Ling, according to the American officials. The officials spoke on the condition of anonymity in order to discuss a delicate diplomatic matter that has already complicated an arrangement made in April between the Department of Homeland Security and China's Ministry of Public Security.
Under that arrangement, signed during a visit to Beijing by Jeh Johnson, the secretary of Homeland Security, the United States would be able to repatriate many of the tens of thousands of Chinese currently in the United States awaiting deportation, some in American detention facilities. In return, the United States would help the Chinese track down wealthy fugitives from China living in the United States who might also be breaking American laws.
Several American officials confirmed that Mr. Ling is in the United States, but they would not say publicly whether Mr. Ling had applied for asylum or give information about his whereabouts. The Department of Homeland Security, which handles asylum cases, does not comment about specific cases because of privacy laws.
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China's Foreign Ministry did not comment after being sent a faxed request for information on Mr. Ling's case. Press officers for the White House, State Department and Department of Homeland Security declined to comment.
Three telephone numbers that people in California used to contact Mr. Ling all had Dallas area codes. Mr. Ling, whose English is said to be poor, did not respond to text messages in Chinese requesting an interview. Two of the three numbers are no longer in service, and no one answered the third number.
Christopher K. Johnson, a former C.I.A. analyst focusing on China, said the Chinese leadership might want Mr. Ling's assistance in prosecuting his older brother. And, Mr. Johnson said, it would want to prevent the "treasure trove" of knowledge he has about Chinese politics from passing to United States officials.
"The leadership would want this guy badly," Mr. Johnson, now at the Center for Strategic and International Studies in Washington, said in a telephone interview. "There's no question that he would have access to a lot of interesting things."
While it is unclear how much Ling Wancheng knows, the Communist Party itself has revealed some tantalizing clues about his brother Ling Jihua's behavior, claiming that his corruption was a family affair. Last month, the party announced that Ling Jihua — a loyalist to the previous president, Hu Jintao — had been expelled from the party and would be tried, saying that he had "accepted huge bribes personally and through his family."
Ling Jihua, 58, rose through the Communist Party's Youth League under Mr. Hu in the 1980s and eventually served as either deputy or chief of the Central Committee's General Office from 1999 to 2012. He was Mr. Hu's personal secretary and closest protégé, and his position came with great powers: the ability to control the guards who protected the senior leadership, a significant voice in top personnel appointments and a central role in carrying out policy.
"It's really the nerve center for the entire system," Joseph Fewsmith, a professor at the Pardee School of Global Studies at Boston University who focuses on Chinese politics, said of Ling Jihua's former position. "This is the essence of power politics."
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Ling Jihua was expected to advance to the elite Politburo, as every person who previously held that position since 1942 had done, including former Prime Minister Wen Jiabao.
But on March 18, 2012, Ling Jihua's son was killed when the black Ferrari he was driving crashed in Beijing. One of two women with him in the car later died.
Ling Jihua's botched cover-up of the episode helped lead to his political downfall. He was denied a spot on the Politburo, demoted to a less important post and, in December 2014, officially put under a corruption investigation.
But the corruption inquiry into Ling Jihua goes far beyond the Ferrari crash, and his younger brother, Ling Wancheng, may have played an important role.
As a senior official, Ling Jihua had his moves monitored. But his brother, as a private citizen, was far less constrained. He built a fortune as the chief of a Beijing-based investment company, which bought well-timed stakes in companies that went on to hold successful initial public offerings, earning the firm $225 million, according to a report in Caixin, a respected Chinese news media company. A company using the same California address that he used to buy his home in Loomis also bought at least two golf courses, one near Loomis, the other in Carson City, Nev., property records show.