The three men were investigating Ganzhou Huajian International Shoe City Co.'s factory in Jiangxi province.
The news comes amid a broader crackdown in China on independent groups pushing for better labor rights in the world's second-largest economy.
Li said China Labor Watch asked police about the Deng and the two other investigators, Li Zhao and Su Heng, on Monday but received no reply. Mr. Hua and two other labor activists had been undercover at two Chinese factories that make shoes for Ms. Trump and other brands.
Night-shift workers at the Huajian shoe factory in Dongguan, China, on September 14, 2016.
Li said the men had documented excessive overtime, with working days sometimes stretching longer than 18 hours, and a base salary below minimum wage. A White House spokesman didn't immediately respond to a request for comment.
As if political news couldn't get any more shocking, Ivanka Trump now finds herself in the middle of a potential scandal in China. Two other shoe brands under the company's umbrella - Marc Fisher and Kylie + Kendall - were also being made at the Huaijian factories, according to China Labor Watch.
Li said investigators had seen Ivanka Trump-brand merchandise, as well as production orders for Ivanka Trump, Marc Fisher, Nine West and Easy Spirit.
The third investigator, Su Heng, had worked at a related factory in the city of Ganzhou in Jiangxi but went incommunicado after May 27.
Former Panamanian Dictator Manuel Noriega Dies At 83
The next day Noriega was transferred to Miami for prosecution that would be sentenced to 40 years in prison for drug trafficking. Ezra Angel, Noriega's lawyer, said the former strongman's three daughters would not issue any public statements.
The activist, Hua Haifeng, who was working on behalf of the advocacy group China Labor Watch, was detained on suspicion of illegal eavesdropping, his wife, Deng Guimian, said late Tuesday.
An activist investigating working conditions at a supplier for Ivanka Trump's fashion brand in China has been detained, his NGO said Wednesday, putting a fresh spotlight on Chinese business activities linked to the USA presidential family. She resigned from her management role at the clothing and accessories company when she took a job in her father's administration, but she still owns part of the business.
Deng said the news came as a shock. Police say Jordan punched his girlfriend as she was driving a van carrying him and her four chi. In his opinion, they are being kept by the police or the factory, so that "there is no access to them".
Associated Press researcher Fu Ting contributed from Shanghai.
Walt Disney Co. stopped working with a toy maker in Shenzhen a year ago after the group exposed labor violations. Since 2015, dozens of activists and lawyers have been detained, arrested and harassed.
"The eagerness of members of the family to do business in China while airbrushing very troubling human rights and labor rights records of the country is troubling", said Nicholas Bequelin, East Asia director for Amnesty International. "With the tightening economic situation, authorities could be looking to protect local manufacturing industries at all costs, even if that includes detaining activists bringing up labor issues".
"In 17 years (since our organization was founded), we have done countless such investigations and have never had any problems", Li said, noting that he suspects the reprisal may be related to the brand they were investigating. Police say Jordan punched his girlfriend as she was driving a van carrying him and her four children along an IN highway Sunday, May 28, 2017, causing a single-car crash that killed her 6-year-old daughter.