HONG KONG (AP) — Five men associated with a Hong Kong publisher known for books critical of China’s leaders have vanished one by one in the last three months, alarming activists and deepening suspicions that mainland authorities are squeezing free expression in the enclave.

The mystery took another turn Tuesday when the wife of the latest man to disappear said she now believes he went to China voluntarily and has canceled a missing person’s report for him.

Lee Bo, a British citizen who vanished Dec. 30, purportedly wrote to say he went to mainland China to help with an investigation. His case has sparked fears that he was seized in Hong Kong by security agents from the mainland and taken there in violation of an agreement giving Hong Kong a high degree of control over its own affairs.

Lee’s wife said she believed the letter showed he wasn’t acting under pressure.

“I believe that it was voluntarily written, so that’s why I retracted the case,” Choi Ka-ping told reporters in brief comments.

Lee and the other four missing men are associated with the publisher Mighty Current, which specializes in gossipy books on political scandals involving China’s Communist leaders and other sensitive topics that are banned in the mainland.

The disappearance of the five — all since October — has raised concerns 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. The principle maintains civil liberties in Hong Kong that are nonexistent on the mainland, including freedom of the press.

British Foreign Secretary Philip Hammond, on a visit to Beijing, said he pressed officials for information on Lee.

“We have urgently enquired, both of the Hong Kong authorities and of the mainland Chinese authorities, what — if anything — they know of his whereabouts,” Hammond said. He added that if Lee is involved in any investigation, it should be settled by the Hong Kong judicial system.

Chinese Foreign Minister Wang Yi, speaking at the same press briefing, warned against making “assumptions or meaningless speculations” about Lee, saying that “above all, he is a Chinese citizen.”

When Lee vanished, he reportedly did not have a travel permit for mainland China with him, an indication he didn’t plan to go there that triggered speculation about Chinese security agents abducting him. The four others were last seen either in mainland China or Thailand.

An image of Lee’s handwritten letter was published by Taiwan’s government-affiliated Central News Agency late Monday and subsequently by Hong Kong media.

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 on the investigation to which it refers and raised more questions than it answered.

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

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

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 bookstore 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.

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Associated Press writers Isolda Morillo in Beijing and Karl Ritter in Stockholm, and news assistant Dong Tongjian in Beijing contributed to this report.