On Friday, New York-based blogger Wen Yunchao, also known as Bei Feng, said his brother and parents were also taken away in southern Guangdong province, after he tweeted a link to the letter.
Both men, who regularly publish openly critical articles, said they had nothing to do with the anonymous letter.
The incident underlines how China's security services are determined to silence critics - even if they live outside the country -- especially if they write in Mandarin and take on China's increasingly autocratic president.
It may also show how sensitive the Party has become to talk of internal rifts.
The wave of detentions also follow last year's disappearances of five Hong Kong booksellers, including a Swedish citizen apparently abducted from Thailand and a British citizen seemingly kidnapped in Hong Kong: Their apparent "crime" to publish gossipy books critical of senior Party figures, including Xi himself, and - allegedly - to export those banned books into mainland China.
The booksellers have effectively now been silenced.
British citizen Lee Bo appeared back in Hong Kong last week after more than two months in mainland China, and was quoted by a Chinese website as saying he would never run a bookstore again or sell "fabricated" books.
Swede's Gui Minhai has been paraded on state television, making an apparently forced confession in relation to a decade-old drink driving case. On Sunday, the South China Morning Post reported that Hong Kong airport is planning to cut back on the number of bookstores at its terminals, eliminating Singapore-based chain Page One and bringing in mainland chain Chung Hwa.
But the writers may be tougher to silence.
Chang, whose real name is Zhang Ping, had a distinguished career a a journalist and editor in China, before being demoted and finally fired from Southern Metropolis Weekly in 2011 for work deemed "inappropriate."
In an article on the U.S.-based China Change website, Chang strongly condemned the Communist Party's attempts to influence foreign media and the "barbaric kidnappings" of his relatives.
"I've always done what I think is right, and have always been willing to accept whatever fate brings as a result of that," he wrote. "The harassment and threats of the authorities allow me to see even more the value of my writings, and encourage me to work harder in future."
Although Chang's sister has now been released, Chang said this was merely an attempt to use his family as hostages to negotiate for the withdrawal of the articles. "I can't take the blackmail," he wrote in an email Monday. "I have to go on."
It is not entirely new for China to put pressure on the relatives of exiled dissidents - the tactic has been used in recent years against blind "barefoot lawyer" Chen Guangcheng after he fled to the United States in 2012, against Miss World Canada titleholder and Falun Gong practioner Anastasia Lin last year, and against Washington-based Radio Free Asia reporter Shohret Hoshur.
But it does show how China's crackdown on free speech is spreading across the globe.
Pursuing critics, China reaches across borders. And nobody is stopping it.
"It seems the authorities will stop at nothing to silence those outside their borders who they can no longer fully control," said William Nee, China researcher at Amnesty International in Hong Kong. "It bears repeating that the persecution of family members of dissidents is a draconian and unlawful tactic that makes a mockery of China's claims to respect the rule of law."
Just as the detention of the Hong Kong booksellers alienated many residents of the former British territory, so the latest assault on foreign dissidents could also backfire.
"Conducting an aggressive manhunt against anyone allegedly involved in commenting on the letter only serves to put more attention on the letter, giving it a shelf life and a wider audience than it ever would have had otherwise," Nee said.
