Three nonprofit workers investigating labor conditions at a Chinese factory making shoes for Ivanka Trump’s eponymous fashion brand have been arrested or gone missing.

Police arrested labor rights activist Hua Haifeng on suspicion of illegal surveillance, the man’s wife told the Associated Press yesterday (May 30). Hua and two colleagues lost contact with their employer China Labor Watch this past weekend, said Li Qiang, who heads the New York-based NGO. He believes authorities are holding all three men incommunicado. The Ivanka Trump brand declined to comment on the matter when contacted by the newswire.

The activists were investigating a Jiangxi province-based factory owned by Ganzhou Huajian International Shoe City, a subsidiary of the Huajian Group and one of the suppliers for Ivanka Trump’s fashion brand. According to Li, the men had documented excessive overtime and a base salary below minimum wage at the factory, and were attempting to confirm evidence of the illegal use of student interns. One of the activists, Su Heng, had been working undercover as an employee at the factory for a month.

The arrest and disappearances came amid an ongoing crackdown against foreign NGOs deemed a threat to China’s stability and national interests. A law enacted in January requires any such organization operating in China to find a government sponsor, register with the police, and submit annual reports on its financing, among other requirements. As a result, some foreign groups have closed their operations in China, while others are still struggling to navigate the red tape.

“Under president Xi Jinping, the Chinese government has promulgated new laws, as well as increased surveillance, to tighten control over these groups,” said Maya Wang, a China researcher at Human Rights Watch. “The fact that they were investigating Ivanka Trump’s brand likely add to this heightened level of official scrutiny.”

According to Li, CLW has not been registered in China under the new law. Before this past weekend some of the group’s undercover investigations had been stopped by police after being reported by factory owners, but such instances hadn’t led to arrests.

The Huajian Group makes 10,000 to 20,000 pairs of shoes a year for Ivanka Trump’s brand, a fraction of the 20 million pairs it produces every year, AP noted. Repeated calls to Huajian made by Quartz went unanswered today (May 31).

Advocacy groups, foreign or not, are having a rough time in China. An LGBT event in the western city of Xi’an was forced to cancel this past weekend, with the organizers being held by police for eight hours. Last week Lee Ming-cheh, a Taiwanese human rights activist detained a few months ago in the southern city of Zhuhai, was formally arrested on the charge of subverting state power.

Visen Liu contributed reporting.