As a result, the American role has been shrouded in silence. Officials at the embassy in Beijing, the State Department and the White House have declined to comment publicly on Mr. Wang’s contacts with American diplomats or the implications of his whistle-blowing on China’s suddenly turbulent internal politics.

“It would be incredibly foolish for the U.S. to play any public cards in this very messy Chinese family feud,” said Orville Schell, the director of the Center on U.S.-China Relations at the Asia Society. “The U.S. and China urgently need to get along, and if there is one thing the Chinese are neuralgic about, it is when their private affairs get aired before foreigners in an embarrassing way.”

The chairwoman of the House Committee on Foreign Affairs, Representative Ileana Ros-Lehtinen, Republican of Florida, wrote to Secretary of State Hillary Rodham Clinton in February demanding the release of all cables, e-mails and memos related to the case. The circumstances, she wrote, raised question about “what steps were taken to secure U.S. national interests and Mr. Wang’s personal safety.” The State Department has not yet complied.

According to the State Department, the United States cannot simply grant asylum to anyone who walks into a diplomatic compound, given the legal and logistical complications of spiriting someone out of a sovereign nation. Asylum seekers — who typically face persecution for political or religious beliefs — usually apply outside their own nation, whether in the United States or a third country.

There are exceptions, but they are rare.

In June 1989, the Chinese dissident Fang Lizhi walked into the American Embassy in Beijing a day after security forces attacked protesters in Tiananmen Square. The embassy at first resisted, but the administration of President George Bush offered him sanctuary, provoking a standoff that lasted a year until the Chinese allowed him to leave, ostensibly for medical treatment. (Mr. Fang died in Arizona on April 6; he was 76.)

This case, however, differs significantly. Mr. Wang, a vice mayor in Chongqing who had overseen the police before a falling-out with Mr. Bo, is no political dissident. During his years as one of Mr. Bo’s top aides, he had a reputation in Chongqing for ruthless and arbitrary enforcement of the law.