(Reuters) — Facebook Inc. said on Wednesday it had blocked a group of hackers in China who used the platform to target Uighurs living abroad with links to malware that would infect their devices and enable surveillance.

The social media company said the hackers, known as Earth Empusa or Evil Eye in the security industry, targeted activists, journalists and dissidents who were predominantly Uighurs, a largely Muslim ethnic group facing persecution in China.

Facebook said there were less than 500 targets, who were largely from the Xinjiang region but were primarily living abroad in countries including Turkey, Kazakhstan, the United States, Syria, Australia and Canada.

It said the majority of the hackers’ activity occurred away from Facebook and that they used the site to share links to malicious websites rather than directly sharing the malware on the platform.

“This activity had the hallmarks of a well-resourced and persistent operation, while obfuscating who’s behind it,” Facebook cybersecurity investigators said in a blog post.

Facebook said the hacking group used fake Facebook accounts to pose as fictitious journalists, students, human rights advocates or members of the Uighur community to build trust with their targets and trick them into clicking malicious links that would install spying software on their devices.

It said hackers both set up malicious websites using look-alike domains for popular Uighur and Turkish news sites and compromised legitimate websites visited by the targets. Facebook also found websites created by the group to mimic third-party Android app stores with Uighur-themed apps, like a prayer app and dictionary app, containing malware.

Facebook said its investigation found two Chinese companies, Beijing Best United Technology Co. Ltd. (Best Lh) and Dalian 9Rush Technology Co. Ltd. (9Rush) had developed the Android tooling deployed by the group.

The Chinese Embassy in Washington did not immediately return a message seeking comment on Facebook’s report. Beijing routinely denies allegations of cyber espionage.

Reuters was not immediately able to locate contact information for Dalian 9Rush Technology Co. Ltd. A man who answered the number listed for Beijing Best United Technology Co. Ltd. hung up.

Western governments are seeking to hold Beijing accountable for mass detentions of Muslim Uighurs in northwestern China, where the United States says China is committing genocide.

China denies all accusations of abuse and says its camps provide vocational training and are needed to fight extremism.

The United Nations estimates that up to 1 million people, mainly Uighurs, have been detained in the Xinjiang camps.

Facebook said it had removed the group’s accounts, which numbered less than 100, and had blocked the sharing of the malicious domains and was notifying people it believed were targets.