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HONG KONG — The mystery surrounding five missing Hong Kong booksellers specializing in literature critical of China deepened after one purportedly wrote a letter saying he was 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 added they would continue investigating the other cases. They didn't say whether Lee had been located.

Protesters hold up missing person notices of the missing booksellers as they walk towards China's Liaison Office in Hong Kong on Sunday.Anthony Wallace / AFP - Getty Images

Lee and four other people associated with publisher Mighty Current, which specializes in books critical of China's Communist Party leaders, have vanished in recent months.

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.

An image of Lee's handwritten letter was first published by Taiwan's government-affiliated Central News Agency late Monday.

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

It added: "It might take a bit of time. My current situation is very well. All is normal."

The letter gave no further details.

Local 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.

Police walk past missing person notices of Gui Minhai (left), one of five missing booksellers from the Mighty Current publishing house, and Yau Wentian (right), a Hong Kong publisher who was last year jailed for 10 years while preparing to release a book critical of Chinese President Xi Jinping.Anthony Wallace / AFP - Getty Images

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."

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."

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 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" that a Swedish national had disappeared. He declined to name the person, in line with Foreign Ministry policy, but said he was in his 50s.

Britain said that it was "deeply concerned" by reports of the disappearance and possible detention of one of its nationals.

But speaking to reporters at a joint press conference with British Foreign Secretary Philip Hammond, who is in Beijing on a two-day trip, Chinese Foreign Minister Wang Yi called on others not to make "groundless accusations" on the case.

The Associated Press and Reuters contributed.