A Hong Kong bookseller who went missing last week has apparently faxed a vague letter to his wife telling her that "all is normal," a cryptic message that has deepened the mystery surrounding the man's disappearance.

Lee Bo, 65, is one of five men linked to book publisher Mighty Current who have disappeared over the last three months. Lee is a major shareholder in Causeway Bay Books, a popular bookstore known for selling a range of titles banned on the Chinese mainland. The bookstore in run by Mighty Current, which has produced books critical of the Chinese government. The company was reportedly preparing about to launch a book about Chinese President Xi Jinping's love life, focusing on one of his former girlfriends.

"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," read the handwritten note purportedly sent by Lee. The message was published by Taiwan's government-affiliated Central News Agency late Monday.

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

The day after Lee disappeared last Wednesday, his wife, Sophie Choi Ka-ping, filed a police report to report him missing. But after the faxed letter surfaced, Ka-ping suddenly withdrew the report on Monday. She stated that she believes the letter is genuine, and that her husband was not pressured to write it, the South China Morning Post reported.

She also reportedly received a phone call from her husband last Wednesday from a number that appeared to be from Shenzhen, the neighboring city that links Hong Kong with the mainland. According to AFP, Lee said at the time that he was "assisting in an investigation" and did not plan to return soon. AFP reported that Lee was last seen at a Hong Kong book warehouse.

In October, four other men linked with Mighty Current also went missing under suspicious circumstances. Gui Minhai, a mainland-born Swedish national who co-owns the publishing company, disappeared while on holiday in Thailand on October 15. Lui Bo, Mighty Current's general manager, Cheung Jiping, the company's business manager, and Lam Wing-kei, who manages the bookstore, each disappeared separately in late October, after they visited Shenzhen.

Watch the VICE News documentary **The End of the Umbrella Revolution: Hong Kong Silenced:**

The disappearances have stoked fears of mainland Chinese authorities using shadowy tactics that erode the "one country, two systems" formula under which Hong Kong was returned to China after 150 years of British rule.

Speaking about Lee's disappearance, Hong Kong lawmaker James To Kun-sun told the Morning Post that the bookseller "had resisted going to the mainland the whole time [in the past]. Why would he suddenly go to the mainland in his own way?"

A colleague of Lam, the bookstore manager, reportedly said the missing man phoned his wife in Hong Kong in early November to say he was "alright," but that he would be staying for a while in an undisclosed location. The source, identified only by the name Lee, said he believed that Chinese authorities had detained the men.

"I suspect all of them were detained. All four went missing at the same time," he told the Morning Post, adding that phone calls left for Lam went unanswered or the line went dead.

On Tuesday, British authorities weighed in on the disappearances, saying it was "deeply concerned" over the possible detentions of the booksellers, one of whom is British. While the British embassy did not name which man held a British passport, local media speculated that it was Lee.

"We are deeply concerned by reports about the disappearance and detention of individuals associated with the Causeway Bay Books bookstore in Hong Kong," read a statement from the British embassy in Beijing.

"We encourage the Hong Kong SAR government to honor its commitment to protecting the freedom of the press, and we hope the Chinese authorities will continue to make every effort to ensure that the environment in which the media and publishers operate in the Hong Kong SAR supports full and frank reporting," the statement added.

The remarks, referring to the "special administrative region" (SAR) of Hong Kong, came as British Foreign Secretary Philip Hammond began his two-day visit to China and after China's Foreign Ministry warned that it would not tolerate outside interference in the affairs of Hong Kong.

Reuters contributed to this report