Five book industry organizations issued a statement on Thursday expressing concern about the mysterious disappearance of five employees of the Hong Kong publishing company Mighty Current, and its bookstore, the Causeway Bay Bookstore. The disappearance of the employees has resulted in widespread media coverage speculating on their fate.

Mighty Current publishes books that have been critical of the Chinese government, and the missing employees are believed to be in custody of Chinese officials.

In a statement, the American Booksellers Association (ABA), the Association of American Publishers (AAP), the Authors Guild (AG), the European and International Booksellers Federation (EIBF), and PEN American Center (PEN), expressed “extreme concern” over news reports about the situation.

The industry groups went on to urge the Chinese government to explain what has happened to the Hong Kong booksellers, noting that when Great Britain relinquished control of Hong Kong in 1997, China promised to protect freedom of press there for 50 years. The statement said: “The summary abduction of men who are engaged in the publication and sale of books would be a very serious violation of press freedom and would have a profoundly chilling effect on free speech in Hong Kong.”

The groups also asked the U.S. government to investigate the circumstances of the disappearances and to assist in the release of the missing men.