A former CIA officer was sentenced Friday to 19 years in prison for conspiring to provide classified information to China, marking the conclusion of a prosecution that came as the U.S. Justice Department stepped up efforts to counter what national security officials contend is an increasingly persistent Chinese campaign to obtain U.S. government and business secrets.

Jerry Chun Shing Lee, who resigned from the CIA in 2007 after more than a decade as a case officer, admitted in May that he had communicated with agents of China's spy service between 2010 and 2018 and prepared documents in response to their requests for information. As part of his plea deal, prosecutors dropped charges related to retaining classified information.

Appearing in a green jumpsuit with "Alexandria Inmate" emblazoned on the back, Lee apologized Friday.

"I let my country down," he said.

Prosecutors alleged Lee was approached in 2010 by a pair of Chinese intelligence officers who said they were prepared to give him a $100,000 cash gift and take care of him "for life" in exchange for information gleaned from his CIA experience. In the following three years, hundreds of thousands of dollars flowed into his personal bank account in Hong Kong.

Questions lingered around Lee's case even as he appeared Friday for sentencing in an Alexandria, Virginia, federal courtroom. His defense lawyers and prosecutors tangled over whether a substantial portion of that money came from the Chinese government. Prosecutor Neil Hammerstrom argued that, for the more than $800,000 that Lee received while receiving requests for intelligence secrets, the former CIA officer must have been providing "really top drawer information."

Lee's defense lawyer Edward MacMahon countered that it was "pure speculation" for prosecutors to claim that the money "must have been for the crown jewels of the CIA." Throughout the case, MacMahon has emphasized that Lee was not charged with actual espionage, arguing that prosecutors offered no evidence that classified information was delivered to the Chinese government.

Prosecutors sought a prison sentence of at least 22 years for Lee, whose defense lawyers suggested a decade-long term. As he handed down a 19-year prison term Friday, U.S. District Judge T.S. Ellis III of the Eastern District of Virginia noted the lingering questions in the case, saying Lee "must have given them something of value" but that "nobody knows for certain precisely what he gave the intelligence officers."

"I do think something of value changed hands," Ellis said.

The U.S. Justice Department has raised alarm in recent years over China's alleged spying efforts, bringing a string of cases related not only to national security information but also trade secrets of American companies. In the most recent of those cases, federal prosecutors Thursday charged Haitao Xiang, a Chinese national and former Monsanto employee, with economic espionage in connection with his alleged attempt to steal intellectual property from the company.

Lee became the third former U.S. intelligence official sentenced to prison this year on charges connected to spying for China.

In May, Ellis sentenced the former intelligence officer Kevin Mallory to 20 years in prison for conspiring to pass defense information to a Chinese government agent. In September, a federal judge in Utah sentenced a former Defense Intelligence Agency officer to 10 years in prison for attempting to provide the Chinese government with classified information.

G. Zachary Terwilliger, the U.S. attorney for the Eastern District of Virginia, said Lee would now join a "wall of infamous traitors."

"He knowingly made a deal with our adversaries out of greed," Terwilliger said.

Prosecutors alleged Lee had compiled information on the locations where the CIA would assign officers. At one point, he sketched a floor plan of an agency facility no longer in use. In 2012, the FBI searched Lee's hotel room in Honolulu and found a day book and planner in his luggage with addresses of CIA facilities and the true names and phone numbers of U.S. assets and covert CIA employees, prosecutors said. Among the assets were "at least eight clandestine sources" Lee handled, prosecutor Adam Small said Friday.

"As a former case officer, everything he knew would have been highly valuable" to China, Small said.