HONG KONG — A Taiwanese man who disappeared during a visit to China is being investigated for suspected activities that “endanger national security,” the Chinese authorities said on Wednesday, the latest twist in a mystery that may dovetail with months of pro-democracy protests in Hong Kong.

A spokesman for the Taiwan Affairs Office of China’s State Council revealed that the man, Lee Meng-chu, was under investigation. Newspapers in Hong Kong and Taiwan reported that he went missing about three weeks ago.

He is thought to have entered the mainland city of Shenzhen, which borders Hong Kong, on Aug. 19. A Taiwanese newspaper soon after published a photograph, which it attributed to Mr. Lee, of armored vehicles near the border.

It was unclear from Beijing’s terse announcement exactly where Mr. Lee was or whether he had been taken into custody.