The New York Times says the investigation took place against a backdrop of a major breach of CIA informants in China that began around 2010.

A former Central Intelligence Agency officer is under arrest on charges of illegally retaining highly classified information relating to the U.S. spy network in China – including notebooks containing lists of informants and details of their operations, the Justice Department announced Tuesday.
Jerry Chun Shing Lee, 53, a naturalized U.S. citizen who now lives in Hong Kong, was taken into custody Monday night at New York’s John F. Kennedy Airport.
An investigation of the former CIA case officer began in 2012, five years after he left the agency, the Justice Department says. He began working at the CIA in 1994 and had a top-secret clearance.
A court affidavit says that in 2012, while Lee and his family were in Honolulu during a layover en route from Hong Kong to the U.S., the FBI executed a court-authorized search of his hotel room. There, agents found and photographed the contents of two small books that …
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