From there, he joined the C.I.A., posing as an American diplomat while serving as a clandestine case officer. From old address records, he appears to have served in Tokyo from about 1999 to 2002. Officials say he also worked at the East Asia Division at C.I.A. Headquarters and the agency’s Beijing station before he left in 2007 and took a job in Hong Kong.
When the C.I.A. noticed in late 2010 that its spies were disappearing, suspicion did not immediately turn to Mr. Lee, according to current and former officials. But as fears of a mole grew, the government set up a secret task force of C.I.A. officers and F.B.I. agents. A veteran F.B.I. counterintelligence agent, Charles McGonigal, was assigned to run it,
former American law enforcement officials said.
As the disappearances continued, analysts concluded that Mr. Lee, even though he had been out of the C.I.A. for years, had known the identities of many of the those who had been killed or imprisoned. He showed all the indicators on a government matrix used to identify potential espionage threats, one former official recalled.
But warning signs can be wrong. At the C.I.A., top officials ruefully remembered the treatment of Brian J. Kelly, an agency officer who in the 1990s was wrongly suspected by the F.B.I. of being a Russian spy. More recently, the Justice Department’s efforts to unearth Chinese spies have suffered embarrassing setbacks, including dropped charges against prominent Chinese-Americans.
In Mr. Lee’s case, other possible explanations existed. Some investigators believed that China had cracked the C.I.A.’s system for communicating with its informants. The spy agency had encountered similar problems in other countries, and some investigators believed the technology was too clunky to stand up to China’s sophisticated computer specialists.
Another group accused C.I.A. officials in Beijing of being sloppy and allowing themselves to be identified when meeting with their informants. It was an acrimonious dispute, and some officials conceded that a combination of factors could account for the damage.
Some former officials who reviewed the evidence described the case against Mr. Lee as strong but circumstantial, not bulletproof. Some at the C.I.A. argued that officials were too quick to suspect a mole — especially a Chinese-American — when there were other explanations.
The F.B.I. was watching in August 2012 when Mr. Lee returned to the United States with his family. Agents secretly entered his hotel rooms in Hawaii and Virginia and discovered two small books with handwritten notes that contained classified information, including the identities of undercover C.I.A. officials, court papers show.
The information the books contained, including details about meetings between C.I.A. informants and undercover agents, as well as their real names and phone numbers, matched documents that Mr. Lee had written while at the C.I.A., according to court documents. It was not clear whether any of the people identified in his documents were part of the Chinese roundup of C.I.A. sources.
Agents spoke with him repeatedly in the following months. Both the attorney general at the time, Eric H. Holder Jr., and Robert S. Mueller III, then the F.B.I. director, were personally briefed on the investigation and pledged whatever resources were necessary. But senior government officials said they cannot recall any serious push to arrest Mr. Lee at the time or to try to charge him with espionage in connection with the lost Chinese informants.
So in June 2013, the agents let Mr. Lee leave. Current and former officials have said that the C.I.A.’s losses had ended by late 2012, so there is no evidence that the decision allowed more informants to be captured or killed.
At least once in recent years, according to a government official, Mr. Lee returned to the United States without attracting the F.B.I.’s attention. It was not clear how or why he did so.https://archive.ph/aUKKP#selection-625.314-625.332