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A Hong Kong student who organized medical support for the city’s pro-democracy protests is missing after being arrested in Mainland China nine days ago, sparking fears that he’s being held for political reasons.

Kwok Chun-fung has not been seen or heard from since he was arrested on Jan. 8 in the southern Chinese city of Guangzhou on charges of “soliciting prostitution,” according to reports.

That’s the same charge that was laid against a British consulate employee who was detained during a business trip to the Mainland city of Shenzhen in August. Simon Cheng, a 29-year-old Hong Kong citizen, said after his release that Chinese secret police had tortured him during his 15 days in detention, accusing him of being a British spy and of fomenting protests in the city.

The similarities between the cases have prompted concerns that Chinese security agents could be using the solicitation charges as a pretext, and that the true purpose of Kwok’s detention is to interrogate him about his support for the protests.

Findcmed, the volunteer medic group that Kwok co-founded to support injured pro-democracy protesters, told Hong Kong media Thursday that the solicitation charges represented an “old tactic,” and that “the truth” would eventually come out. The group said it couldn’t comment further while the proceedings against Kwok were ongoing.

Kwok is a 20-something Hong Konger who studies at the Guangzhou University of Chinese Medicine and Findcmed, which formed on the messaging app Telegram, organizes volunteers to provide medical support for protesters injured in pro-democracy demonstrations, which have frequently involved violent confrontations between police and protesters.

The police have been accused of brutality for their use of tear gas, batons, rubber bullets, water cannons, and bean bag rounds, while Amnesty International has documented more extreme cases of police violence, resulting in injuries ranging from head wounds to broken arms.

Another of the group’s administrators, who uses the pseudonym Little Cow, said in a post on the online forum LIHKG that he was in the middle of a phone call with Kwok, who was in his university dorm room in Guangzhou, when his friend disappeared.

According to Little Cow, Kwok had gone to open the door when the phone dropped to the floor and the line went dead. Little Cow said he had subsequently been told by Kwok’s roommate that his friend was under administrative arrest.

AFP reported Thursday that Kwok’s classmates at the Guangzhou University of Chinese Medicine had been briefed by school officials about his disappearance, telling students that he had been arrested for soliciting a prostitute in a hotel.

The offence is punishable by up to 15 days in administrative detention.

Cheng, the staffer at the British consulate, was held for 15 days before he was released to return to Hong Kong in August. In November, having fled to an undisclosed third country for his safety, he spoke out about his ordeal, saying he had been beaten, blindfolded, chained to a cross, and held in stress positions for hours during his detention.

Cheng, who said he supported the protests in a personal capacity and had monitored them as part of his job, said his interrogators accused him of being a British spy, and pressed him to admit that the British government was behind the pro-democracy protests roiling the city. He said they recorded him reading two letters of apology, one for soliciting prostitution, and the other for betraying the motherland.

Officials in the semi-autonomous Chinese city have failed to halt the youth-led pro-democracy protests that have upended Hong Kong since June, resulting in more than 7,000 arrests. A survey commissioned by Reuters last month found 59 percent of Hong Kongers supported the protests, and more than a third of respondents had attended a demonstration.