Li Zhao, seen here in March 2016, is one of three men arrested after investigating conditions at a Chinese factory that produces Ivanka Trump-brand shoes. (Deng Guilian/via AP)

A labor activist researching working conditions in a Chinese factory that makes shoes for Ivanka Trump’s label has been detained by police, according to his wife and a labor advocacy group, while two others have gone missing and are presumed also to have been detained.

The three activists were investigating labor conditions at Huajian shoe factories for China Labor Watch, a New York-based nonprofit organization that aims to defend workers’ rights.

Huajian has previously said it has been making shoes for the U.S. president’s daughter for nearly a decade, accounting for one-third of her shoes made in China but only a small proportion of its total output.

But China Labor Watch said its investigation in working practices at Huajian factories has apparently been closed down by local police.

Two of the activists were told they were not allowed to leave China in April and May, said Li Qiang, CLW’s founder — something he said was relatively common.

But now all three appear to have been detained, which he said was unprecedented for his investigators.

“This never happened before in my 17 years’ experience. This is the first time,” he said. “The only reason we think this case is different is that this is Ivanka Trump’s factory.”

A White House spokeswoman, Hope Hicks, referred questions to Ivanka Trump’s company, which declined to comment. Huajian spokesman Wei Xuegang said he was not aware of any arrests.

Marc Fisher Footwear, which manufactures Ivanka Trump shoes but does not own these facilities, said: “We were unaware of the allegations, this arrest and will look into them immediately.”

Activists Hua Haifeng and Li Zhao had been investigating labor practices at Huajian factories in Ganzhou city in southeastern Jiangxi province and Dongguan city in southern Guangzhou province, while a third activist, Su Heng, was working undercover inside the Ganzhou factory, CLW’s Li said.

Hua’s wife, Deng Guilian, said she had not spoken to her husband since Sunday afternoon, when he was on his way from Dongguan to Ganzhou. But she said she had received a phone call from the Public Security Bureau in Ganzhou on Monday saying that he had been detained on suspicion of “illegal monitoring.” Police declined to give further details, she said.

Li said the other two activists were missing, after last being seen in Ganzhou, and were not in contact with relatives.

The police in Ganzhou did not respond to requests for information.

China Labor Watch has carried out frequent investigations into labor violations in Chinese factories that make items including Disney toys and Apple iPhones.

In May, it issued an interim report on working conditions at Huajian’s factories, citing long hours and low pay, among other issues. The group said it had written a letter to Ivanka Trump detailing the allegations in late April but had yet to receive a reply.

In that report, it alleged that employees are forced to work at least 12½ hours a day and at least six days a week — at a monthly salary of about 2,500 yuan ($365). It said pay for some workers amounted to the equivalent of less than a dollar an hour.

Li said his investigators had documented long working days, the longest stretching 18 hours, from 7:10 a.m. to 1:30 a.m. He also said workers were given only one or two days off per month during busy periods, while there is no safety training, even though many employees come into contact with oils and glues.

The Washington Post’s own interviews with workers at Huajian in Dongguan earlier this month broadly corroborated those findings, with workers claiming their overtime often exceeded the legal maximum of 36 hours a month. Huajian has not responded to requests for comment.

William Nee, China researcher at Amnesty International in Hong Kong, said the detentions appear to represent another attack on civil society in the country under President Xi Jinping.

“The Chinese state seems to be aggressively going after any activists who have any ties to overseas organizations under the all-encompassing umbrella of ‘national security,’ ” he said.

“President Donald Trump has bitterly complained of unfair trade practices, but now we have a vivid example of workers rights activists in China being detained merely for trying to ensure that labors laws are actually enforced in practice and don’t just exist on paper. He should use his new friendship with Xi Jinping to press for their release.”

This is not the first time that looking into Huajian’s work for Ivanka Trump has reportedly led to problems for Chinese citizens.

Last year, a team from the French news agency Agence France-Presse was given access to the Huaijian factory in Dongguan. But the resulting coverage and photographs, some of which showed workers on assembly lines and living in dark dormitory buildings, apparently did not please the company’s management.

As a result, China’s Global Times newspaper reported, some workers involved in the reporting and shooting of images had been fired. The Global Times blamed “misreporting” by Western news media out to malign the reputation of the Trump family, and quoted the factory chairman as confirming the firings, adding he preferred not to be identified by name.

But a man who described himself as the head of public relations at the factory, but also did not give his name, emerged from the plant on a recent day to deny that anyone had been fired.

The Global Times report also cited staff members from two factories in Guangdong as saying that their companies had received a memo from Ivanka Trump’s China-based agent shortly after the U.S. department store Nordstrom dropped her products in February, notifying them that any media reports on her suppliers in China would not be good for her image, for political reasons.

China Labor Watch said Hua had been stopped by police while trying to visit Hong Kong last week and discouraged from continuing his investigation into Huajian. Fellow activist Li Zhao was also phoned by police in the days before he disappeared, CLW said.

Both men had been conducting an investigation into the use of student labor at the Ganzhou factory on Sunday when they were last heard from.

Foreign Ministry spokeswoman Hua Chunying said China encouraged foreign nongovernmental groups to conduct research in the country as long as their activities were “normal” and obeyed Chinese law.

Hua’s wife said she believed that her husband’s job was “helpful and meaningful to society.”

“If he is sentenced for this, I can’t accept it. I can’t accept it’s justice,” Deng said by phone from her home in the central province of Hubei, adding that she didn’t know how the family would cope if he is imprisoned.

Fighting back tears, she said she had not told Hua’s elderly parents or her own two children, ages 7 and 3, who think their father is working away from home.

“They always ask to video chat with their father. I have to say to them, ‘Your father is very busy,’ and tell them, ‘He will talk to you when he’s not busy.’ ”

Shirley Feng and Congcong Zhang in Beijing and Drew Harwell and Matea Gold in Washington contributed to this report.

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