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.