A former CIA officer was arrested at Kennedy Airport in New York on Monday after investigators believed he upended years worth of United States spying operations in China.

Jerry Chun Shing Lee has been under suspicion by the FBI since at least 2012, which is shortly after the time the CIA's operations in China began to unravel, The New York Times reported. There was growing belief within the CIA that a mole "was exposing its roster of informants," while others believed that the "Chinese government had hacked the C.I.A.’s covert communications used to talk to foreign sources of information."

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Lee finished with the CIA in 2007 and was living in Hong Kong; he was charged with "unlawful retention of national defense information," the Times reported.

The Times elaborated:

Prosecutors said that both before and after he and his family moved back to the United States in 2012, Mr. Lee met with former C.I.A. colleagues and other government employees. As the agency began losing assets in China, it was not clear at first that the losses were systematic, but as the disappearances mounted, the American intelligence community eventually realized it had a major problem. The case had frustrated counterintelligence officials in the F.B.I. and C.I.A. as they sought to determine how the Chinese had disrupted agency operations in the country. The F.B.I. suspected an insider had revealed sensitive information to the Chinese government, a theory not initially embraced by the C.I.A. Mr. Lee eventually became a prime suspect in the hunt for a traitor.

Lee, 53, came back to the U.S. in 2012 to live with family, and his belongings were searched by FBI agents during his hotel stays in Virginia and Hawaii. The federal agents found two small books belonging to Lee that contained handwritten classified information, such as identities of CIA informants and real phone numbers, court papers showed. Prosecutors have said the content "reflected the same information contained in classified cables that Mr. Lee had written while at the agency," the Times reported.

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Between May and June 2013, Lee was interviewed five times by the FBI and never disclosed the two books they had discovered, the Times reported. It's still not clear why Lee wasn't arrested for the notebooks when they were originally found.

More than 12 CIA informants had been killed by the Chinese government in a two-year span, and intelligence operations had been crippled for years after; the Times reported it as a "devastating setback" for the agency.