A longtime CIA case officer who conspired to spy for China was sentenced to 19 years in prison Friday, even as the extent of his crime remains a mystery.

Prosecutors were unable to prove that Jerry Chun Shing Lee, 55, gave classified information to the Chinese but argued it was the only explanation for the more than $840,000 he received while getting requests for CIA names, methods and locations.

“He must have conveyed some classified information to the People’s Republic of China in exchange for payment,” Judge T.S. Ellis III said in sentencing Lee in federal court in Alexandria.

After lying repeatedly over the years to investigators, Lee eventually admitted to getting 21 tasks from Chinese intelligence operatives. He also admitted to keeping a notebook that included names and numbers for at least eight CIA sources. But Lee insisted that he never handed over the secrets he collected for his Chinese handlers.

“I could only say I’m sorry,” Lee said. “I let my country down. I let many people down.”

At a news conference after the hearing, law enforcement officials noted federal prosecutors have charged three Chinese espionage cases this year. They said the Chinese have been particularly aggressive in targeting former clearance holders on social media and may be using information from the 2015 hack of government personnel data.

“When sorrows come, they come not single spies, but in battalions,” said David Burns, a senior official at the Justice Department, quoting Shakespeare.

Some intelligence officials still suspect Lee is responsible for the collapse of the CIA’s spy network in China, when assets began dying or defecting to the Chinese en masse. But there are some who think there may be another explanation for how the CIA’s covert communication system was breached, and law enforcement was unable to prove otherwise.

Prosecutors made no mention of that disintegration in court, focusing instead on the risk that the release of Lee’s information posed, which was designated “secret.” They said the CIA would conduct a damage assessment once the case was concluded, under the assumption that everything Lee gathered was given to the Chinese.

“For this amount of money, the [Chinese] must have been getting really top-drawer, high-quality national defense information,” Assistant U.S. Attorney Neil Hammerstrom argued. “We put all the pieces together. . . . He sold out his country for money.”

Prosecutors said Lee lied to investigators about the finances of a tobacco company he co-owned with a man he knew to have ties to Chinese intelligence, falsely claiming high-profile clients including Gucci and the British Consulate. When told the firm looked like a front for illegal activity, Lee replied: “I hear you, but I’m not going to say anything.”

Defense attorneys emphasized that Lee was never charged with actual espionage.

“The government described their worst possible nightmare,” said defense attorney Nina Ginsberg, but “has no proof that it was caused.”

It’s “pure speculation . . . a bridge too far” to say the money “must have been for the crown jewels of the CIA,” said fellow defense attorney Ed MacMahon.

U.S. Attorney G. Zachary Terwilliger said at the news conference that these cases are “exceedingly difficult” because spies like Lee “know how to cover their tracks.”

At the same time, he acknowledged that intelligence information is often kept out of criminal court. “There is a push-pull about what it is we’re able to use” without “tipping our hands to our adversaries,” he said.

In letters to the court, family and friends expressed shock and disbelief at Lee’s crime, describing him as a patriotic and modest family man.

“While he was at the CIA, he was considered an exemplary employee,” Ginsberg said.

A naturalized U.S. citizen from Hong Kong and an Army veteran, Lee served as a CIA case officer from 1994 to 2007. After briefly working for a Japanese tobacco company and getting fired, in 2010 he started his own tobacco firm with a former Hong Kong detective with ties to Chinese intelligence.

At a private dinner that year in Shenzhen, China, Lee was approached by two Chinese operatives who said they knew about Lee’s work for the CIA in China and offered to take care of him “for life” in exchange for national defense information, starting with $100,000 and a cellphone.

Lee reported the approach to an old CIA colleague — but not the offer of payment for information.

For the next few years, Lee compiled secret information requested by the Chinese operatives, including the locations that officers with certain experience were assigned and the time and location of a sensitive operation.

In early 2012, Lee’s former CIA colleague urged him to reapply to the agency, part of a ruse to bring him back to the United States for questioning. Lee had several interviews with the CIA over the next year, during which he lied about getting instructions from the Chinese and keeping notes that included classified information.

During one trip he took to the United States, FBI agents searched Lee’s hotel room and found a notebook that included intelligence from CIA assets, assets’ true names, and some locations and numbers.

Ginsberg said “it took the CIA months of referring to secret cables going back years to figure out who these people were.” The sources named, she said, were not in China.