The third arrest in one year of a US official suspected of helping Chinese spies has bared the tense battle between the two superpowers' intelligence agencies.
The arrest late Monday by US authorities of former Central Intelligence Agency agent Jerry Chun Shing Lee was reportedly linked to Beijing's brutal dismantling five years ago of the CIA's network of undercover operatives and informants inside China.
That followed the June 2017 arrest of a former State Department security officer, reportedly also an ex-CIA official, Kevin Mallory, on allegations that he handed over US secrets to Chinese agents for $25,000.
Three months before that, a China-based US diplomat, Candace Claiborne, was charged for taking tens of thousands of dollars in cash and gifts from Chinese intelligence.
According to The New York Times, US counter-intelligence has been working overtime since at least 2012 to uncover a possible pro-Beijing mole within the ranks of America's espionage services.
The Times reported last year that starting in 2010, to the end of 2012, the Chinese uncovered and killed "at least a dozen" sources the CIA had inside China and imprisoned six or more others. One of them was shot in front of his colleagues to send a message, the Times reported.
That debacle severely damaged the US government's ability to collect secret information on its Asian rival.
It also implied that the huge Chinese Ministry of State Security and the espionage arms of the People's Liberation
Army had been able to penetrate US intelligence.
Lee, 53, was arrested late Monday after he landed in New York.
A naturalized US citizen who had lived for the past several years in Hong Kong, he served in the US Army in the 1980s and spent 13 years from 1994 at the CIA, where he had top secret clearance.
The charge against him was limited to one count of unlawful retention of national defence information. But the details of an investigation spanning at least five years suggested much more.
The indictment said that in 2012 FBI agents had secretly examined Lee's luggage while he was travelling.
They found two notebooks jammed with classified information including the identities of CIA covert agents and assets, notes from their meetings, locations of covert facilities, and phone numbers.
The charge did not say whether this information, which would have been extremely valuable to Beijing, had been provided to the Chinese, or whether they had gained access to it otherwise.
Citing intelligence community sources, the Times reported that Lee was one of those eyed for years by investigators as a possible mole.
Growing espionage threat
US officials would not comment on the newest case, but the government has been increasingly worried about Beijing's spying.
"The United States faces a large and growing threat to its national security from Chinese intelligence collection operations," said a November 2016 report of the US-China Economic and Security Review Commission.
In 2008 two US defence officials were arrested for passing secret materials to a US citizen originally from Taiwan, Tai Shen Kuo, who officials said was working for Chinese intelligence.
In 2010 agents discovered that an incoming 28-year-old CIA recruit, fluent Mandarin speaker Glenn Shriver, was already on the payroll of the Ministry of State Security, and had been tasked to infiltrate the agency.
The most recent arrests may be tied to the probe into the CIA's loss of its China network.
Claiborne, arrested in March 2017, had worked in China for several years for the State Department. There she became close to two men, at least one of whom she knew was from the Ministry of State Security who supplied her with gifts of cash and money, court documents say.
But she spent much of the money on another man, a person she lived with in Beijing and Shanghai who was described as half her age, the documents allege.
The criminal complaint against her says only that Claiborne provided the Chinese agents with nonclassified information.
Mallory, of Leesburg, Virginia, was arrested in June 2017. The defence contractor and former State Department security officer, 60, was also a fluent Chinese speaker. The Washington Post reported he had actually worked for the CIA before going to the private sector.
His indictment described frequent, knowing contacts with Chinese agents promising them valuable defence materials.
"Your object is to gain information, and my object is to be paid," he told them in one message.