China has been increasingly successful in recruiting former U.S. intelligence officers to pass along government secrets, a trend that is getting more attention as the Trump administration attempts to limit Beijing’s spy abilities.

At least four Americans have pleaded guilty or were convicted of espionage or attempted spying on behalf of the Chinese over the past year. Kevin Mallory, who was convicted of espionage charges and lying to the FBI, was sentenced to 20 years in prison Friday.

Mallory, 62, spent more than 20 years in intelligence, first as a covert case officer for the CIA and later as an intelligence officer for the Defense Intelligence Agency. Prosecutors alleged he gave his Chinese handlers documents of a classified spying operation and a CIA analysis of a foreign country’s intelligence capabilities.

Mallory was a rare example of a spying case going to trial. Since March, three other Americans have accepted plea agreements: Ron Hansen, 58, a former intelligence officer for the Defense Intelligence Agency; Jerry Chun Shing Lee, 54, a former CIA case officer; and 63-year-old Candace Claiborne, a former State Department employee.

Top U.S. officials have warned of China’s long-term, multipronged campaign to advance Chinese interests at the cost of American national security.

“No country poses a broader, more severe intelligence-collection threat than China,” FBI Director Christopher Wray told the Council on Foreign Relations last month. “They’re doing it through Chinese intelligence services, through state-owned enterprises, through ostensibly private companies, through graduate students and researchers, through a variety of actors all working on behalf of China.”

For decades, Beijing focused on pressuring Chinese nationals and individuals with Chinese heritage living in the U.S. to collect information on its behalf. More recently, China appears to have taken a page out of Russia’s playbook by paying U.S. intelligence personnel to provide them with private and classified information. In two of the more high-profile cases, the Soviet Union and later the Russian Federation paid former FBI agent Robert Hanssen and former CIA counterintelligence officer Aldrich Ames to spy for them.

Jeff Asher, a former CIA counterintelligence analyst, said money has historically been a motivator in many instances of former intelligence officers turning against the U.S. “A lot of times when you get former intel officers, it's a financial thing or it's a vengeance thing,” he told the Washington Examiner. “Whether or not that's exactly what the Chinese are using, it wouldn't be surprising. Using financial resources to get spies is as old as the profession itself."

David Charney, a psychiatrist who has interviewed some of the most well-known spies, suggested this problem will not be going away any time soon. "There's always going to be a few [government employees] that are struggling with their lives and things are not going right and they have bad things happen, too much for them to handle, and they would be the vulnerable ones, where there would be a risk like that. The risks are always there," Charney told the Washington Examiner.

When Mallory was contacted by a Chinese headhunter via LinkedIn in early 2017, he was running his own consulting firm and struggling financially after leaving his government job in 2012. Mallory accepted $25,000 from the Chinese amid his money troubles, a low figure compared to Lee and Hansen.

In 2010, Lee, a naturalized U.S. citizen born in Hong Kong, was approached by the Chinese who offered to pay him $100,000 in cash and “take care of him for life,” according to the Justice Department. Intelligence officials suspect Lee compromised CIA assets in China, some of whom were killed, though they have been unable to prove he was the mole. Lee, who fit the demographic profile for those historically targeted by Chinese intelligence, left the CIA in 2007 and worked in the tobacco business in Hong Kong but was unsuccessful.

Hansen was deep in debt and living off his $1,900-month pension when he began meeting with Chinese intelligence officers in 2014. Hansen attended conferences about intelligence issues in exchange for at least $800,000 over the years.

It does not appear Claiborne was in financial distress when she accepted tens of thousands of dollars of gifts over five years from China’s intelligence service in exchange for internal State Department documents, though prosecutors alleged she was motivated by the financial rewards.

These cases coincide with the U.S. increasingly sounding the alarm on China’s espionage abilities. On Wednesday, President Trump declared a national emergency that effectively bans companies from using Huawei, China’s largest telecom company, out of concern that it could be used to spy on Americans.

Testifying to the Senate Judiciary Committee in December, the FBI’s top counterintelligence official said China is the “the most severe counterintelligence threat” facing the U.S. today. “Every rock we turn over, every time we looked for it, it’s not only there, it’s worse than we anticipated,” said Bill Priestap, who served as the assistant director of the FBI’s counterintelligence division until 2018.

China is also suspected of being behind the hack of Marriott’s Starwood chain hotel reservation system, a breach that exposed the personal data and travel details of as many as 500 million people, as well as the 2015 data breach of the Office of Personnel Management, the human resources department for the federal government.

Former and current officials say China is playing the long game.

“With China, they don’t need a quick fix," LaRae Quy, who worked for the FBI until 2006 on Russia and Chinese counterintelligence, told the Washington Examiner. "They’re in it for the long haul. Years can go by before they actually will then push further for the next step."