China has detained a Taiwanese man on state security charges after he reportedly distributed photos of Chinese troops massing equipment on the Hong Kong border.

Lee Meng-chu was being investigated after he “allegedly engaged in illegal activities that endanger state security”, said a spokesman for mainland China’s Taiwan Affairs Office, without elaborating.

Friends and family feared Lee – a volunteer activity organiser in a small fishing community in southern Taiwan – had been detained in China after they were unable to contact him for weeks.

Chen Yalin, a Taiwanese mayor, told Taiwan’s government-run Central News Agency earlier that Lee sent him a photo showing paramilitary troops gathering, and that he was unable to contact Lee after that.

Chen added that he was in touch with Lee on 20 August, when Lee said he was at the Hong Kong side of the border with China and wanted to cross over to Shenzhen. “The People’s Liberation Army were gathering at the border, the situation seemed tense,” Chen quoted Lee as saying.

Chen said Lee – from the township of Fangliao – was expected to fly from Hong Kong to Indonesia to meet him in Jakarta for a conference in late August but had failed to arrive.

The news of Lee’s detention came as Hong Kong was mired in its worst political crisis for decades, with anti-government protests showing no sign of abating after three months. Photos of drills conducted in the Chinese border town of Shenzhen have fuelled speculation that China might mobilise its military to crack down on protests in Hong Kong.

People held by China on state security charges are often detained for months at an unknown location and are denied visits from lawyers and families.

In March 2017, Taiwanese rights activist Lee Ming-che disappeared on a trip to China and emerged in September at a court hearing in the southern city of Changsha. He was later sentenced to five years in prison for the “subversion of state power” for holding online political lectures and helping the families of jailed dissidents.