And in August, reports attributed to American intelligence officials asserted that Pakistan had allowed Chinese experts to inspect the remains of the stealth helicopter that crashed during the May mission to kill Osama bin Laden. Although Pakistan and China denied the reports, Beijing would have a great interest in examining the tail of the Black Hawk helicopter, the part of the aircraft that was not destroyed by the Navy Seals team, to learn more secret details of American stealth technology.

Meanwhile, the mystery of the leaked W-88 warhead design remains unsolved. At first, the American government suspected that Wen Ho Lee, a Los Alamos nuclear scientist, had leaked the W-88, but it produced no evidence that he had done so. He was held in solitary confinement for nine months, eventually pleaded guilty to one count of mishandling classified information and won an extraordinary apology from the federal judge who presided over the case.

Misled by the Energy Department, the F.B.I. had chased the wrong person for three years. Finally, in 1999, Robert Bryant, then the bureau’s deputy director, enlisted Stephen Dillard, a veteran counterintelligence agent, to head a major investigation of how China had acquired the design of the W-88.

The inquiry was led by the F.B.I. and run by a task force of 300 investigators from 11 federal agencies, including the Defense Department, the C.I.A., the National Security Agency and the Defense Intelligence Agency. On Sept. 11, 2001, some of the investigators were killed when American Airlines Flight 77 was flown by terrorists into the Pentagon.

But the investigation went on. Mr. Dillard’s task force, operating out of public view, looked at the nuclear weapons laboratories, government agencies and defense contractors in California and several other states who had manufactured parts of the warhead. The F.B.I. interviewed the walk-in, who was by now living in the United States, but he could shed no light on the source of the document.

Finally, after four years, the investigation ended with American intelligence agencies no closer to knowing how China obtained the secret design of the nuclear warhead. The answer remains locked up in Beijing.

More than a decade later, China’s spies continue to conduct espionage against military targets. Last year, a Pentagon official was sentenced to prison, the last of 10 people rounded up by the F.B.I., all members of a loosely connected Chinese spy network on the West and East Coasts that was run by Lin Hong, a spymaster in Beijing. The data that made its way to China included information on the Navy’s Quiet Electric Drive, designed to make submarines harder to detect, the B-1 bomber and projected American arms sales to Taiwan.