The mystery surrounding five missing Hong Kong booksellers known for titles banned in mainland China deepened after one purportedly wrote a letter saying he was fine and helping with an investigation on the mainland, prompting his wife to drop a missing person's report.

Hong Kong police said late Monday that Lee Bo's wife canceled the report, but that they would continue investigating the other cases. They didn't say whether Lee had been located.

Lee is among five people associated with publisher Mighty Current, which specializes in books critical of China's Communist Party leaders, who have vanished in recent months. One of the five is British, with Hong Kong media reporting that it is Lee.

Their disappearances have prompted fears that Beijing is eroding the "one country, two systems" principle that's been in place since Britain ceded control of Hong Kong to China in 1997, granting the city civil liberties nonexistent on the mainland, including freedom of the press.

When Lee vanished last Wednesday, he reportedly did not have his travel permit for mainland China with him, triggering speculation that Chinese security agents entered Hong Kong to abduct and spirit him there.

Four other people linked to the publishing company went missing in October, but they were last seen either in mainland China or Thailand.

An image of Lee's handwritten letter was first published by Taiwan's government-affiliated Central News Agency late Monday. Hong Kong media have also published the image, crediting the news agency.

The letter, faxed to an employee at the publishing company's Causeway Bay Bookstore in Hong Kong, said: "Due to some urgent matters that I need to handle and that aren't to be revealed to the public, I have made my own way back to the mainland in order to cooperate with the investigation by relevant parties."

"It might take a bit of time," it said. "My current situation is very well. All is normal."

The letter gave no details about what the investigation was related to.

Britain's Foreign Office responded to inquiries about media reports that Lee is British by saying in an email Tuesday that one of the people reported missing is a British citizen, without naming the individual. It said it was "deeply concerned" about the case and has "urgently requested" help from authorities in Hong Kong and mainland China to find out the person's location and condition.

Hong Kong media reported that Lee's wife, Choi Ka-ping, asked police to drop the missing person's report after learning of the letter, the authenticity of which could not be independently confirmed. Choi's phone number was written on the fax, but calls to her by The Associated Press went unanswered.

Choi told Hong Kong's Apple Daily newspaper that she found his travel permit for mainland China at home after he went missing.

Hong Kong pro-democracy lawmakers and human rights activists were skeptical the letter proved Lee was safe.

"If he did indeed write the letter, it was almost certainly written under duress," said William Nee, Amnesty International's China researcher. "What we see in mainland China all the time is that police and state security put enormous pressure on family members not to speak to media and not to raise a fuss on social media. If indeed it was state security that detained Lee Bo, one wonders whether the same tactics are being used to silence family members here in Hong Kong."

China's nationalist newspaper Global Times slammed the bookshop in an editorial Monday for "profiting on political rumors" and selling books with "trumped-up content."

"Although the Causeway Bay Bookstore is located in Hong Kong, it actually stays in business by disrupting mainland society," the paper said.

Hong Kong police still have missing person's files open for three other staff members or shareholders of the publisher or the bookstore. A fifth person, Gui Minhai, a Swedish national who is one of the publishing company's owners, went missing in Thailand in October, according to Hong Kong media and human rights groups.

Swedish Foreign Ministry spokesman Joakim Edvardsson said Monday that the government was "very concerned" about the disappearance of one of its citizens.