Mainland Chinese authorities have said they are investigating a Taiwanese man who went missing after entering Hong Kong and reportedly crossing the border into Shenzhen last month. Ma Xiaoguang, a spokesman for the mainland Taiwan Affairs Office, confirmed at a press conference in Beijing on Wednesday that Lee Meng-chu was being investigated for “activities that endanger state security”, without disclosing further details. In Taipei, the government of President Tsai Ing-wen expressed grave concern over the detention of Lee and called on the mainland authorities to provide a full account in addition to allowing Lee’s family to visit him on the mainland. “We are deeply concerned about the detention of our national Lee Meng-chu in China and have asked the Mainland Affairs Council and relevant agencies to negotiate in full force with the Chinese side about the issue,” the Presidential Office said in a statement on Wednesday. The Mainland Affairs Council, the island’s top cross-strait policy planner, said the mainland authorities must “provide detailed information about where he was detained and why he was restricted of his personal freedom in line with the agreement on cross-strait joint efforts to fight crimes”. “It must also officially inform the relatives of [Lee] and swiftly make arrangements for his relatives to visit him on the mainland in the presence of lawyers to uphold his judicial rights,” said Chiu Chui-cheng, the vice-chairman of the council. He said the council would remain in contact with Lee’s family to offer all necessary aid. Taiwan’s Straits Exchange Foundation, which deals with the mainland, said it had sent three requests to its mainland counterpart, the Association of Relations Across the Taiwan Strait, for help in locating Lee since he was reported missing on August 20. Beijing targets Taiwanese writer for supporting Hong Kong protesters Lee was feared to have been detained after friends and family said they were unable to reach the volunteer activity organiser from southern Taiwan after his entry to Hong Kong on August 18. Shenzhen police had previously told Lee’s family that they did not have a record of him entering Shenzhen despite claims by Lee’s friends that he had dined with them in the city, which borders Hong Kong. Earlier Taiwanese media reports suggested Lee’s family and friends had been unable to contact him since August 20, after he was scheduled to leave Hong Kong for Shenzhen. Lee reportedly distributed photos of mainland Chinese troops massing equipment just outside Hong Kong, where there have been 14 weeks of anti-government protests triggered by a now-withdrawn extradition bill that proposed the transfer of criminal suspects to the mainland. Lee, who works for a volunteer group in Fangliao township, a fishing community in Pingtung county, entered Hong Kong on August 18, Taiwan’s government-run Central News Agency reported. He apparently sent photos to his brother and to Chen Ya-lin, the head of Fangliao township, showing paramilitary troops and equipment on Hong Kong’s border with mainland China, the agency said. On Wednesday, Chen said he and Lee’s family had tried through various channels to find out what had happened to him. “But the initial response from Shenzhen in China said Lee had never entered the city and told us to ask Hong Kong,” he said. “What the Taiwan Affairs Office said indicated that Lee did enter Shenzhen and got ‘investigated’.” Taiwan’s refugee bill does not apply to Hong Kong protesters for now, Taipei says Chen said Lee had been missing for more than three weeks and the mainland had been reluctant to disclose any information. “We have no idea whether he has been tortured and forced to admit to any crime,” he said, adding Lee was a businessman and not a politician. “Though he has been enthusiastic about public affairs, he has never engaged in politics, and what China did to him is not only unreasonable but also shocking,” Chen said. He called on international organisations and democratic countries to look at the matter closely and condemn it. Lee’s family had hired a lawyer, and both the Mainland Affairs Council and the Straits Exchange Foundation were helping the family to secure Lee’s release, Chen said. Taiwan’s Democratic Progressive Party on Wednesday condemned Beijing for arresting Lee under the excuse of violation of national security, saying what the mainland did would only create further resentment among the Taiwanese public and make them more fearful about visiting the mainland.