BEIJING — A liberal Chinese writer living in Germany has said security officers in China detained three members of his family in connection with a mysterious online letter that denounced the iron-fisted rule of President Xi Jinping.

The writer, Chang Ping, said on Sunday in a post on China Change, a pro-democracy website, that two younger brothers and a younger sister were “abducted by the Chinese police” and were the “latest victims” in official investigations of the letter, which was posted this month and demanded that Mr. Xi resign.

The detentions bring to 11 the number of people who have been held by security officers in a widening investigation of the origins of the letter. One freelance writer, Jia Jia, was released on Friday after being detained on March 15.

Mr. Chang wrote an article for Deutsche Welle on March 19 that criticized Mr. Jia’s detention. He also gave an interview to Radio France Internationale, or RFI.