The Chinese government “systematically dismantled” CIA spying operations in China starting in late 2010 and killed or imprisoned at least a dozen CIA sources over the next two years, according to a media report.

The New York Times in its report cited 10 current and former US officials, who described the intelligence breach as one of the worst in decades. They spoke on condition of anonymity.

The report said US intelligence and law enforcement agencies scrambled to stem the damage, but were bitterly divided over the cause of the breach. Some investigators were convinced there was a mole within the CIA, while others believed the Chinese had hacked the covert system the CIA used to communicate with its foreign sources. The debate remains unresolved, the paper said.

The CIA, which declined to comment to the Times, also declined to comment on Saturday to The Associated Press.

The number of CIA assets lost in China rivaled those lost in the Soviet Union and Russia as a result of the betrayals by both CIA officer Aldrich Ames and FBI agent Robert Hanssen, who were arrested in 1994 and 2001, respectively, the report said.

As many as 20 CIA sources were killed or imprisoned in China over a two-year period, the Times said, citing two former senior US officials.

Investigators suspected a former CIA operative of being a mole, but failed to gather enough evidence to arrest him and he is now living in another Asian country, the report said.

Those who rejected the mole theory attributed the losses to sloppy American tradecraft in China.

By 2013, the FBI and CIA concluded that China no longer had the ability to identify American agents, the Times said.