Taiwan launched an inquiry into several officers suspected leaking of military secrets to China, a case that highlights the espionage threat the democracy faces from its much larger neighbor.

The Defense Ministry in Taipei said in a statement Wednesday it had found evidence that military personnel, including a lieutenant colonel, gave sensitive information to representatives of China’s ruling Communist Party.

Active and retired members of the army’s aviation and special forces commands were suspects in the leaks, the Taipei-based Liberty Times reported earlier, without saying where it got the information. Two people were detained and four others were released on bail, it added.

Taiwan’s military faces a constant struggle to ferret out spies linked to China. The US — Taiwan’s biggest military backer — has long been worried about the self-ruled island’s ability to keep tech and other secrets out of Beijing’s hands.

See: Taiwan Holds Military Officers Suspected of Spying for China

Late last year, Taiwan’s Defense Ministry said that China’s spying posed a “serious threat.” Then in January, Taiwan detained three active-duty officers and a retired Air Force officer on suspicion of spying. Last month, Taiwan detained five more people over allegations they recruited officers to collect intelligence for China, which has pledged to bring Taiwan under its control someday, by force if necessary.

The Defense Ministry said Wednesday that it will step up education of its personnel about China’s infiltration efforts, while ramping up security. Alex Huang, deputy secretary-general to President Tsai Ing-wen, said at a briefing in Taipei that the latest leaks were “shameful and should be strictly punished.”