A former CIA case officer was sentenced Friday to 19 years in prison for conspiring to provide American intelligence secrets to the Chinese government, in an espionage case that some current and former officials say dealt a devastating blow to U.S. intelligence operations.

Jerry Chun Shing Lee, 55, served 13 years as a Central Intelligence Agency case officer in several locations overseas, including China, where prosecutors said he had firsthand knowledge of some of the agency's most sensitive secrets, including the names of covert CIA officers and clandestine human sources in China.

But many aspects of the case remain a mystery. Lee was never charged with actually giving any secrets to the Chinese. While the Justice Department told a federal judge that "it is all but certain" he did so, his lawyer said that "the government has offered only conjecture as a basis for these claims."

Although the extent of his cooperation with the Chinese is apparently unknown, the case illustrates how aggressively China works to get its hands on U.S. secrets. Lee is the third former U.S. intelligence officer to be convicted in less than a year of conspiring with the Chinese to give them national defense information.

Lee pleaded guilty in May to a single of charge of conspiracy to provide national defense information to a foreign government. He was arrested nearly two years ago after FBI agents searched his hotel room and found notebooks and a thumb drive containing the names and phone numbers of covert CIA employees and informants, details of a sensitive CIA operation, and information about covert facilities.

Lee admitted that in 2010 he met in Shenzhen with two Chinese intelligence officers who offered to pay him and "take care of him for life" if he would provide secrets he learned as a CIA officer. Over the coming months, he said, the Chinese gave him at least 21 separate requests for intelligence secrets.

A month after that meeting, Lee deposited $17,000 in cash into his bank account, even though he was operating a failing consulting business at the time. Federal prosecutors told Federal District Court Judge T.S. Ellis that the Chinese gave him a total of $840,000, but Lee's defense lawyer, Edward MacMahon, said the government has never actually linked the deposits to Chinese intelligence officers.

"It's speculation that this money was for the crown jewels of American intelligence," MacMahon told the judge.

Prosecutor Neil Hammerstrom conceded that the intelligence community can never be certain of exactly how much Lee disclosed — saying only that "it is all but certain" that he did pass along sensitive secrets that endangered the lives of intelligence sources and severely hampered the CIA's intelligence-gathering abilities in China.

"Had the government ever possessed proof that Mr. Lee gave any classified information to the Chinese, it certainly would have charged him with the actual transmission of national defense information," MacMahon said in court filings. But Lee never made any such admission. MacMahon said the government told Lee's lawyers that the intelligence community never did a damage assessment.

"The government has posited no direct evidence that Mr. Lee actually caused any harm to the United States," MacMahon said.

But Ellis concluded that it was "more likely than not that at least some portion of the money came from the Chinese, so he must have given them something of value."

Some current and former U.S. intelligence officials said Lee's agreement to cooperate with the Chinese in 2010 came at the same time the CIA's covert communication system was compromised. For that reason, they say, it's impossible to know whether Lee or the system failure caused the most damage.

Even so, Frank Figliuzzi, a former FBI counterintelligence official and NBC News national security contributor, said Lee's efforts represented "a horrific loss for the intelligence community, and it's not a loss than can be recovered from easily."