Simon Cheng, 28, who works as a Commerce and Investment Officer at the British Consulate General in Hong Kong, has been placed under administrative arrest for 15 days, Chinese Foreign Ministry spokesman Gen Shuang said on Wednesday.  "The person is not a British citizen. He is a Hong Kong citizen, so he is a Chinese citizen and this is a purely Chinese domestic affair," Gen said.
Cheng sent a message to Lee on the night of August 8, shortly before he crossed the border into Hong Kong, Lee told CNN.
"Simon has signed a contract with the British government," she told CNN. "If he had not been given this assignment, he would not have needed to go to Shenzhen. The UK must take responsibility for saving Simon."
The British Consulate General in Hong Kong on Tuesday addressed local media with details of Cheng's disappearance. "We are extremely concerned by reports that a member of our team has been detained who is returning to Hong Kong from Shenzhen," an email spokesman said.
"We are supporting his family and seeking additional information from the authorities in Guangdong province and Hong Kong," the spokesman added.
The British Embassy in Beijing made an identical statement. to say he was on the high-speed train. Later, he messaged to say that he was about to cross the border, according to screenshots seen by CNN. If Cheng was on the high-speed train between Shenzhen and Hong Kong, the only the immigration point is at West Kowloon Station, in some parts of the station but Chinese law applies, even though she is in Hong Kong, operating under a separate continental legal system.
China does not recognize dual citizenship for any Chinese national, which means that it does not recognize BNO passports.
Cheng's disappearance comes as Hong Kong approaches its 12th weekend of massive pro-democracy protests. Travelers told CNN that security at the Hong Kong-China border has increased since the start of mass protests in June.
A Hong Kong man, who recently arrived in a city in China, told CNN that border police appear to be picking people up at random to get additional searches. The man who asked not to be baptized said police scrolled through photos on his phone.
In a statement posted online on Wednesday morning, Cheng's family reiterated that Cheng was in Shenzhen on a "business trip."  On August 9, the family contacted the Hong Kong police and immigration department and were told that they could travel to mainland China on their own to report the case to the authorities there, the statement said. On August 10, the Immigration Ministry said the Cheng family was "administratively detained" but had no information as to why, where and for how long he would be detained.