"Hua Haifeng, Li Zhao and Su Heng must be released if they are being held exclusively for investigating possible labour abuses at factories making shoes for Ivanka Trump's label", said William Nee, China researcher at Amnesty International. But conditions at the factory remained the same, Li said. "But now it has become more political".
The Chinese government has stepped up its clamping down on independent labor groups in recent years.
The man is now accused of illegally using the interception equipment, according to Li Qiang, the CEO from China Labor Watch. In the past, the worst thing Li feared was having investigators kicked out of a factory or facing a short police detention. The president of her brand, Abigail Klem, said the company could not respond to the watchdog's claims because they were provided with no specific evidence, but that they complied with labor regulations in China.
An Ivanka Trump representative declined to comment.
The detentions underscore concerns that Trump family businesses could be receiving special treatment as Beijing hopes to curry favor with the US president. Li also said that the accusations against the detained investigator - that he was suspected of illegally using eavesdropping equipment - had "no factual basis", according to the Associated Press.
The father-of-two was working undercover with the two other men, Li Zhao and Su Heng, in Jiangxi province. One of the plants is located in the city of Dongguan in southern Guangdong province, the other is in Ganzhou.More news: Apple unveils iOS 11, Siri speaker and new iPad Pro at WWDC
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He said the activists also found that the factory employed "student laborers". The parent company is known as Huajian Group. "Police told me that my husband has been detained for illegal monitoring activities", she said. "If your official letter contains a stamp and a signature, we can confirm whether the media is real or not".
Li says he investigated the Huajian shoe factory precisely because it was making shoes for the Ivanka Trump brand and he thought he could leverage that to help the workers.
A spokeswoman for Marc Fisher Footwear, which manufactures Ivanka Trump shoes, said the firm was unaware of the allegations and would look into them immediately.
The focus of the investigation was Huajian Group, which Mr Li said had violated labour codes on overtime, minimum wage, verbal abuse of workers and violations of womens' rights.
She said she was notified of his arrest, but not where he was being held, on Tuesday in a phone call from a Jiangxi public security bureau. 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. I feel his work is legal and meaningful. Rights groups and journalists have documented widespread abuse of the system over the years.
Nicholas Bequelin, East Asia director for Amnesty International, says the "eagerness of members of [the Trump family] to do business in China while airbrushing very troubling human rights and labour rights records is troubling". Later he was contacted by Shenzhen National Security Brigade asking for a meeting. She said she did not know their current whereabouts.