Then on Saturday, Mr. Gui vanished again.
Mr. Gui was sitting on a train bound for Beijing, accompanied by two diplomats from the Swedish Consulate in Shanghai, Ms. Gui said. As the train neared Beijing, plainclothes police officers boarded at a station and led Mr. Gui away. His daughter said she did not have details of what happened, and did not know whether he or the diplomats resisted.
“I just know that things have taken a very drastic turn for the worse,” said Ms. Gui, who has led a campaign to win her father’s freedom.
“This group of about 10 men in plain clothes just came in and grabbed him from the train and took him away,” she said. “I presume it must have been quite a scene.”
When he was taken off the train, Mr. Gui had been traveling to the Chinese capital for a medical examination at the Swedish Embassy, after he showed symptoms of amyotrophic lateral sclerosis, also called A.L.S. The disease attacks the brain and spinal cord, progressively impairing actions like walking, swallowing and using chopsticks.
Last year, Liu Xiaobo, the imprisoned Chinese dissident who won the Nobel Peace Prize, died under police guard in a Chinese hospital after he was belatedly found to have liver cancer.
“Gui Minhai is exhibiting symptoms of a serious neurological condition, symptoms that did not exist before he was taken into custody in October 2015,” said John Kamm, an American businessman and human rights advocate who has been working with Mr. Gui’s family to try to win his release to go abroad for treatment. “I pray we will not witness the death in prison of another person accused of political crimes.”
Renewed detention of Mr. Gui could rekindle strains between China and Sweden and its European allies, unless he is soon released. On Monday, the Swedish Embassy in Beijing would not comment, referring questions to the Swedish Ministry of Foreign Affairs in Stockholm. It declined to comment when contacted.
The Chinese Ministry of Foreign Affairs said that it “had no understanding of the situation you referred to,” and suggested asking another ministry. The Ministry of Public Security did not answer questions faxed on Monday.
Mr. Gui, 53, was born in eastern China, went to study in Sweden in 1988 and became a citizen of the country in 1992. But in recent years, he worked out of Hong Kong, where he became a co-owner of Mighty Current Media, a small publishing house.
Supporters call the detention of Mr. Gui and the four other Hong Kong booksellers a campaign by the Chinese government to shut down publishers of books offering unsparing criticism and also unflattering gossip about the party elite. Hong Kong, a former British colony attached to mainland China, became a center for producing Chinese-language books that were banned in China, but that mainland readers often bought and smuggled back.
The mysterious detentions magnified worries in Hong Kong that China has grown dismissive of the legal guarantees that are intended to protect the city, which was returned by the British in 1997, from interference by the mainland authorities.
But Mr. Gui’s latest disappearance suggests that at least some in the Chinese security forces remain unbowed by the criticisms of the secretive detentions. His daughter said she was unsure which arm of the Chinese security or police forces boarded the train. In recent months, he had visited the Swedish Consulate in Shanghai several times without incident, she said.
After Mr. Gui was taken away, Chinese officials told Swedish diplomats that he was suspected of sharing secret information with Swedish diplomats and of meeting them illegally. Ms. Gui said she was baffled by what secrets her father could have known, and did not see how it could be unlawful for her father to meet a Swedish diplomat.
“Obviously, they’ve acknowledged that he’s a Swedish citizen,” Ms. Gui said of the Chinese police. “How that is a crime is difficult to comprehend.”