Two Hong Kong booksellers released from mainland Chinese custody after going missing went straight back to China after returning home to ask police to drop their missing persons cases, a Hong Kong newspaper reported.

The two, Cheung Chi Ping and Lui Por, were among five Hong Kong booksellers who have disappeared over the past six months and re-surfaced in mainland Chinese police custody.

Their cases have raised concern in Hong Kong that Chinese authorities are overriding a "one country, two systems" formula protecting Hong Kong's freedoms since its return to mainland China from British rule in 1997.

All five were from Causeway Bay Books, a Hong Kong book shop that specializes in gossipy political books about Chinese leaders.

Cheung and Lui were released on separate days last week and traveled to Hong Kong to ask police to drop their cases. Hong Kong's South China Morning Post newspaper, citing unidentified sources, said both then crossed back into mainland China on the same days.

A spokeswoman from the Hong Kong police declined to comment.

Telephone calls seeking comment from the Hong Kong and Macau Affairs Office, the Chinese government agency that oversees issues regarding the two former European outposts, went unanswered.

Lui did not answer his telephone and the other booksellers could not immediately be reached.

The reported swift return of the two to mainland China after asking Hong Kong police to drop their cases is likely to fuel speculation about whether they are acting freely or not.

Many people in Hong Kong believe the booksellers, who have appeared in a series of confessions and interviews on Chinese television, were abducted by mainland agents.

Mainland Chinese police said in a statement last week that Cheung, Lui and colleague Lam Wing Kee were being investigated but would be released on bail.

It was not clear what would happen to their two other colleagues, Gui Minhai, who is a Swedish national and went missing while in Thailand, and Lee Bo, who is a British national and went missing from Hong Kong.

The South China Morning Post said Cheung had told immigration officers at the border checkpoint that he was in a hurry. He declined to give police a formal interview or to give an official statement, the newspaper said.

This week, a series of at least 10 emails reviewed by Reuters showed that bookseller Lee Bo had expressed fears that Gui had been taken by Chinese agents for "political reasons" before he, himself, went missing.