The police grabbed Mr. Cheung 11 days after the Telegram channel was created, accusing him of administering it. They also accused him of posting a guide on how to assassinate police officers. Mr. Cheung denies the allegations, and a New York Times search could not find posts matching what the police described.
Mr. Cheung, a skinny 29-year-old, was grabbed at a mall around noon on July 18, according to his account. Four plainclothes officers waited for him to unlock his phone and then jumped on him, trying to pry it out of his hands.
After the officers tried to use his face to unlock the phone, they took him to a police station, where, he said, he was roughed up and interrogated. Later, officers went to his home and used a USB drive loaded with hacking software to break into his computers, according to his account of the incident. He said that he had been held for more than 10 hours and that he was not sure how the police had identified him.
Hong Kong police confirmed the investigation, but they declined to comment further on it.
The police may have been motivated by the facial-recognition tool, which Mr. Cheung said he had showed off in a Facebook video he posted last month. Making use of Google technology, Mr. Cheung, a college dropout who studied computer science, built an algorithm to identify police officers based on a small collection of photos that had been posted online. He said he did not continue to pursue the project because of a lack of time.
Mr. Cheung said his detention had cemented his fears. He said the plainclothes officers who arrested him did not identify themselves until they reached the police station. Later, an investigator in a suit urged him to open his phone as a way of demonstrating his innocence “to the 42nd floor” — a phrase Mr. Cheung said seemed to refer to high-ranking police officials. He did not believe that the police ultimately gained access to the phone, although they did break into his other devices.
The police also did not initially allow him to make a call. Only when he said he planned to play Ping-Pong with his uncle did they relent and let him. He said he contacted a friend instead, adding “I hate sports.”
Mr. Cheung also said he believed he had been followed by plainclothes officers since his arrest. When he arrived an hour late to an interview with The Times, he said it was because he was trying to lose a tail. With the help of his black Tesla, he said, he managed to outrace whoever it was on the highways of the New Territories in northern Hong Kong.