BEIJING —  Chinese security officers appear to have detained several people in their investigation of a mysterious, fiery letter posted online this month that strongly criticized President Xi Jinping and demanded that he resign.

Four of those detained work for Wujie News, or Watching, the state-run website that posted the letter on March 4, the first day of an annual political conclave in Beijing, according to a person tracking the inquiry who asked not to be identified. They are the two top editors and two technicians.

In addition, Wen Yunchao, a Chinese activist living inNew York, said in a telephone interview that his parents and younger brother in southern China had been missing since Tuesday, after police officers and officials warned his parents that Mr. Wen should tell them what he knew about the letter. Mr. Wen said he had nothing to do with distributing the letter on the Internet, and so refused to bow to the demands.

Another person detained by the police was Jia Jia, 35, a freelance writer who recently finished a stint as a visiting scholar at Sun Yat-sen University in the southern city of Guangzhou. Mr. Jia, a friend of Mr. Wen, was detained on March 15 as he prepared to board a flight to Hong Kong, which he was accustomed to visiting.

Mr. Jia’s family members and friends say he had nothing to do with the letter. On Friday night, a post appeared on Mr. Jia’s WeChat social media account that implied he had been freed. “Thank you, everyone,” it said.

Mr. Jia is also a friend of Ouyang Hongliang, editor in chief of Wujie. People with knowledge of the situation have said Mr. Jia called Mr. Ouyang to ask him to take the letter off Wujie after he noticed it circulating online. Mr. Ouyang is one of the website employees who have been detained.

The website began operations in September, and its main investor is the Communist Party committee of the western region of Xinjiang. Other investors include Caixun, which owns the financial magazine Caijing, and Alibaba, the e-commerce giant.

Wujie was started to report on news of Mr. Xi’s economic plan of “One Belt, One Road,” aimed at helping China increase investment and trade across Asia and Europe, said Qiao Mu, a journalism professor at Beijing Foreign Studies University.

The future of Wujie and its 100 employees is uncertain. It suspended publishing articles after the letter appeared, but it again began posting news on Friday, although only articles written by the main state news organizations.

The widening investigation by security officers suggests that they have so far been unable to pinpoint the origins of the letter but are under pressure to do so.

The letter, signed by “Loyal Communist Party Members,” was sent by email to people with ties to China around the time it appeared on Wujie, shortly after 12:01 a.m. on March 4. Bill Bishop, the editor of Sinocism, a China newsletter, said he received it in his Gmail inbox at 10:37 p.m. on March 3 in the Washington, D.C., area, where he lives. “Strange,” he said.

On Twitter, Mr. Wen, the activist, urged President Obama to ask Mr. Xi to release his parents and brother. “He kidnapped them on March 22,” Mr. Wen wrote. Mr. Xi is expected to visit the United States next week for a summit meeting on nuclear security.

Mr. Wen said in the interview that his sister-in-law had told him that his parents and his younger brother, Wen Yun’ao, a driver for a local government, were all missing. Mr. Wen said his sister-in-law had given no details of when or how his parents disappeared but had said Wen Yun’ao, her husband, was taken away by officials.

Starting this month, Mr. Wen said, the police and officials repeatedly visited his father, Wen Shaogan, 71, and mother, Qiu Xiaohua, 64, at their home in Jiexi County, Guangdong Province, and told them that Mr. Wen had to admit to helping spread the letter.

“At the start, they said they wanted to know if I had anything to do with the open letter calling for Xi Jinping to resign,” Mr. Wen said. “But on the 17th, they said directly that they knew I hadn’t written the letter but believed I had something to do with spreading it. They promised that if I told them who wrote the letter and passed it on to me, and how I spread it around, then I would not be held culpable and it would not be held against my family. Otherwise, they said, my younger brother might lose his job.”

Mr. Wen, a vocal critic of the Chinese government who is also known by the pen name Bei Feng, said he had passed on a message to the officials through his parents that he had nothing to do with writing or distributing the letter.

“I told them very clearly that I could not admit to something that had nothing to do with me,” Mr. Wen said. “I told them very clearly that I didn’t write the letter and had not helped anyone to distribute it, and I had not issued the letter on any websites.”

Two police officers reached by phone in Jiexi County on Friday refused to answer questions about Mr. Wen’s family or to pass on a number for a press officer.

“Engaging in this form of collective punishment, which we thought was only something done in feudal dynasties, is absolutely egregious for a country that claims it’s adhering to the rule of law and respecting human rights,” said William Nee, a China researcher at Amnesty International.

“We demand that the authorities immediately release Wen Yunchao’s family members, who had nothing to do with this,” he said. “And they should release any other people who are in detention related to this letter.”