A Communist Party newspaper says Chinese police are investigating a detained Hong Kong bookseller for allegedly selling books by mail illegally in mainland China, the latest twist in a case that has raised concerns over erosion of civil rights in Hong Kong.

The Global Times said Monday that Gui Minhai had admitted to selling more than 4,000 books by mail despite knowing they had not been approved by China's publications authority.

Gui, who holds Swedish citizenship, was one of four people connected with the Mighty Current publishing house who went missing in October. Gui resurfaced in January, making a tearful appearance on Chinese state TV to say he surrendered for fleeing the country in violation of the terms of his suspended sentence over a 12-year-old fatal drunk driving case.