A labor-rights group whose investigators have been detained by Chinese authorities is appealing to Ivanka Trump to help secure their release after the men probed factories that supplied shoes for her brand.

In a four-page letter sent Tuesday to Trump at the White House, where she is an adviser to her father, President Trump, China Labor Watch said the three undercover activists were targeted because they were investigating a supplier connected to the Ivanka Trump brand.

“In the past 17 years, China Labor Watch has conducted hundreds of factory investigations and yet this is the first time our investigators face criminal detention,” Li Qiang, executive director of China Labor Watch wrote in the letter, the first page of which was seen by Bloomberg. “We believe the reason to that is pertaining to the supplier factories of your brands particularly.”

The three activists were detained last month while investigating labor conditions at supplier Huajian Group in the southeastern province of Jiangxi. China Labor Watch said that it has been investigating conditions at 15 factories making products for the Ivanka Trump brand in the past year.

In the letter, Li implores Trump to use “your influence to help us advocate for the release of our investigators.”

Li also sent additional information on labor conditions at the factory, he said. Li said he could not yet confirm if the White House had received the letter as of Thursday.

The White House staff was not immediately available for comment after hours, while a representative for Ivanka Trump did not immediately respond to a request for comment.

The U.S. State Department this week called for the release of the three men, while China’s foreign ministry spokeswoman Hua Chunying rebuffed the appeal and said the case was an “internal matter.” White House spokesman Sean Spicer said Tuesday the U.S. will “continue to exert the proper diplomatic pressure on that.”

“After discussions with our licensee, we have determined that Ivanka Trump brand products have not been produced at the factory in question since March,” Abigail Klem, president of the Ivanka Trump brand, said in a statement on Wednesday. “Our licensee works with many footwear production factories and all factories are required to operate within strict social compliance regulations.”

The men – Su Heng, Hua Haifeng and Li Zhao – have confessed to illegally obtaining industrial secrets for a foreign organization, a state-run media outlet, The Paper, reported this week. They are accused of using recording devices, including a watch to take photos, in a factory in Ganzhou city and taking internal documents in exchange for payment. The trio were detained for illegal eavesdropping, said Wen Yu, a lawyer representing Hua.

This is the second letter the labor organization has sent to Trump, after one in April that alleged a number of labor violations at two unnamed factories that supply the Ivanka Trump brand.