Last Updated Jan 17, 2018 6:54 PM EST
WASHINGTON -- A former CIA officer arrested formay be the key to a years-old mystery at the spy agency.
As early as 2010, officials at CIA headquarters in Langley, Virginia, were concerned about a mole. CIA contacts and informants in China were being killed or imprisoned -- and investigators wanted to know why.
Now, sources say former CIA case officermay have been leaking information to the Chinese.
Lee worked at the spy agency from 1994 to 2007 and worked on cases in China. According to court papers, in 2012, FBI agents discovered he had handwritten notes containing the "true names" and "phone numbers" of assets, covert CIA employees and CIA facilities.
FBI agents found the documents when they searched two hotel rooms Lee stayed in during a trip to the United States. He was then interviewed five times, but, for unknown reasons, he was not arrested and returned to Hong Kong.
Lee, a naturalized U.S. citizen, served in the Army before his time at the CIA. He most recently worked at Christie's auction house in Hong Kong. On Monday, he flew to JFK Airport in New York, where he was arrested.
Former CIA deputy director Michael Morell, CBS News' senior national security contributor, says Lee's case could prove as damaging to U.S. intelligence as Soviet moles Aldrich Ames and Robert Hanssen were during the Cold War.
The FBI writes that at the time of his arrest on Feb. 21, 1994, Ames was a 31-year veteran of the CIA, who had been spying for the Russians since 1985.
"They way you caught Ames, the way you caught Hanssen was you caught them in the act of sharing classified information," Morell told CBS News.
Hanssen was arrested and charged on Feb. 18, 2001 with committing espionage on behalf of the intelligence services of the former Soviet Union and its successors, according to the FBI's website. He was considered "the most damaging spy in FBI history."
"If the reporting here is true, with regard to the loss of Chinese assets, and if the reporting is accurate with regard to Mr. Lee's role in that and if that was intentional on his part, then this is the equivalent of Ames and Hanssen."
Lee was not charged with espionage, but he is accused of illegally retaining classified information. If convicted of the charges, he faces up to 10 years in prison. CBS News was unable to reach a lawyer for Lee.