Hong Kong leader Leung Chun-ying said on Monday there was no indication so far outside agents were involved in the disappearance of five booksellers specializing in publications critical of China, but that it would be "unacceptable" if any were.
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.