HONG KONG: Hong Kong leader Leung Chun-ying said on Monday he was “highly concerned” about the disappearance of five booksellers specializing in publications critical of China, while playing down speculation one may have been abducted by Chinese agents.

The disappearances have stoked fears of mainland Chinese authorities using shadowy tactics that erode the “one country, two systems” formula under which the former British colony has been governed since its return to Chinese rule in 1997.

Lee Bo, 65, a shareholder of Causeway Bay Books, “vanished” last week, according to a missing person report filed by Lee’s wife on Thursday. Four associates involved in publishing or selling literature critical of Beijing have also gone missing in mysterious circumstances over the past few months.

Hong Kong opposition lawmakers protested on Sunday outside Beijing’s representative office over Lee’s disappearance.

“We are highly concerned with this case,” said Beijing-backed Hong Kong leader Leung. He added there was “no indication” so far that Lee may have been kidnapped by Chinese State Security agents in the financial hub and whisked across the border to China, though investigations were ongoing.

“If mainland (Chinese) law enforcement personnel enforce the law in Hong Kong, this is unacceptable because it breaches the Basic Law,” Leung said, referring to the city’s mini-constitution, in rare comments defending Hong Kong’s autonomy against over-reaching by Beijing.

The Basic Law guarantees wide-ranging personal freedoms, including broad freedom of speech, and independent law enforcement.