A former Taiwanese professor missing since August last year has been detained in mainland China, the authorities confirmed on Wednesday. Shih Cheng-ping, 56, is the third Taiwanese man whose detention has been confirmed in recent months. In September the mainland authorities announced that businessman Tsai Jin-shu and volunteer activity organiser Lee Meng-chu were also being held. Ma Xiaoguang, a spokesman for the Taiwan Affairs Office, told a press conference in Beijing on Wednesday that all three were being investigating for “activities endangering national security”, but denied there was anything unusual about their cases. “The mainland has always followed the law in cracking down on criminal activities that endanger national security. It is normal law enforcement. There isn’t a period of time where the [number of detainees] appears to be a lot or abnormal,” he said. Hong Kong bookseller Lam Wing-kee who fled to Taiwan says he’s ready to reopen in Taipei The detentions come at a time of heightened cross-strait tensions, with restrictions having been placed on travel for mainland residents since July. Taiwan is due to hold a general election in January 2020, where President Tsai Ing-wen, from the independence-leaning Democratic Progressive Party, is seeking a second term in the face of a challenge from Han Kuo-yu, from the mainland-friendly Kuomintang. Shih was a professor at National Taiwan Normal University and a former Kuomintang member. After retiring from his university post he worked as an economist for mainland state-owned enterprise Huaxia Group and was a frequent cross-strait traveller until his disappearance in August last year. However, Taiwanese media only reported that he was missing earlier this month. The Taipei-based newspaper Liberty Times reported that sources who knew Shih had disclosed that his family had wanted to keep a low profile and did not want to seek help through official channels such as the island’s Mainland Affairs Council. The sources told the newspaper that the family was worried that he would become another Lee Ming-cheh, the Taiwanese rights campaigner jailed for five years for subversion two years ago. Beijing extends sweeteners for Taiwanese weeks before Taipei election The Straits Exchange Foundation (SEF), a semi-official Taiwanese organisation set up to handle exchanges with mainland, said it had received 149 reports of Taiwan residents disappearing on the mainland since May 2016. Of those, 101 cases have been resolved, but 48 are still outstanding. On Wednesday, Taiwan’s Mainland Affairs Council said that the mainland authorities had not notified Taipei of Shih’s detention, adding that a similar thing had happed in the case of Tsai Jin-shu and Lee Meng-chu. Tsai, the chairman of the Federation of Southern Taiwan Cross-Strait Associations, was last seen in July last year, when he travelled to Fujian province for a food fair , according to the SEF. Meanwhile, Lee lost contact with his family and friends after arriving in Hong Kong in August this year. He had planned to cross the border into mainland China and Taiwanese media later reported that he had distributed photos of mainland Chinese troops massing equipment in the city of Shenzhen. “We urge the mainland authorities to provide due legal process and fully protect the rights of the [detainees]. If there are other Taiwanese residents in similar situations, please notify us and their families as soon as possible,” the Mainland Affairs Council said.