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.

The three activists were placed under investigation and told they were not allowed to leave China back in April, said Li Qiang, CLW's founder - something, he said, was relatively common.

But they now all appear to have been detained, he said, something he said was very rare.

"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."

White House spokeswoman Hope Hicks referred questions to Ivanka Trump's company, which declined to comment. Huajian did not immediately respond.

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 reports that the Huajian factories in Ganzhou city in Jiangxi province used student labor, while a third activist, Su Heng, was working undercover inside the factory, CLW's Li said.

Hua's wife Deng Guilian said she had not spoken to her husband since Sunday. But she said she had received a phone call from the Public Security Bureau in Ganzhou on Monday saying he had been detained on suspicion of "illegal monitoring." Police declined to give further details, she said.

Li said Hua had been accused of using "eavesdropping equipment." The other two activists are also unreachable, he said.

China Labor Watch has carried out frequent investigations into labor violations in Chinese factories making anything from Disney toys to Apple iphones.

In May it issued an interim report on working conditions at Huajian's factories, citing long hours and low pay among others issues. The group said it had written a letter to the first daughter 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. next day.

In January, Liu Shiyuan, then spokesman for the Huajian Group, told the Associated Press the company makes 10,000 to 20,000 pairs of shoes a year for Ivanka Trump's brand - a fraction of the 20 million pairs the company produces a year.

This is not the first time 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 another Huaijian factory in the southern city of 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 media out to malign the reputation of the Trump family, and quoted the factory chairman as confirming the sackings, 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 factory on a recent day to deny that anyone had been fired.

The Global Times report also cited staff 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 due to political reasons.

Hua's wife Deng said she believed 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," she said by phone from her home in the central province of Hubei.

She said she had not told her two young children, ages 7 and 3, who still 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.' "