U.S. intelligence officials remain concerned that China has compromised the methods that the CIA uses to communicate with overseas informants, dampening agents’ enthusiasm after the Monday arrest of a former CIA officer for allegedly retaining classified information, The Washington Post reported.

Between 18 and 20 key CIA sources in China went dark from 2010 to 2012, in what US officials described as one of the worst intelligence failures in decades, according to the New York Times.

Investigators are examining the possibility that China compromised the informants by intercepting their communications with CIA handlers, the Post reported.

Among their considerations: Covert U.S. communications with informants have been intercepted by other countries with less sophisticated hacking capabilities than China, according to The New York Times.

And investigators noted that the CIA has been sloppy in its communications with Chinese informants, including by allowing informants to know their handlers’ identities, the paper said.

Additionally, the CIA initiated an effort within the past few years to revamp its covert communications, the Post reported, suggesting that the agency has long been concerned about such intercepts.

The CIA did not respond to the paper in the report.

The Times reported that former CIA case officer Jerry Chun Shing Lee, 53, became a prime suspect in the purge of U.S. informants in China, even as others theorized at the time that vulnerable lines of communication may have been the culprit.

Lee was booked Monday for illegally retaining top secret information whose disclosure could cause “exceptionally grave damage to the National Security of the United States,” an FBI agent wrote in the affidavit supporting his arrest.

That information included the “true names and phone numbers of assets and covert CIA employees,” the FBI claims.

But Lee, whose security clearance was terminated when he left the CIA in 2007, has not been charged directly in the loss of the informants.

In fact, veteran U.S. spies and senior officials are skeptical that Lee will ever be charged for any crime relating to the apparent collapse of the CIA’s network in China, multiple outlets reported.

The FBI could have charged Lee with illegally retaining classified information when it interviewed him in 2013, the New York Times reported, but let him return to China to allow the investigation to continue.