Another Chinese dissident in exile has said Chinese authorities have detained his relatives as part of the ongoing investigation into an open letter calling for the resignation of the president, Xi Jinping.

Chang Ping – whose real name is Zhang Ping – said his two brothers and a sister were “abducted” by Chinese police on Sunday near his father’s home in Sichuan province.

“The police showed no legal warrant for detaining them,” wrote Chang in a column on Sunday on China Change, a US website that focuses on Chinese human rights issues. “I have not been able to contact my family members directly, but through other sources I’ve learned that the police asked my family to contact me and demand that I immediately cease to publish any articles that criticise the Chinese Communist party.”

In an email on Monday, Chang said his sister had just been released but his two brothers were still in detention. Chinese authorities, he said, were attempting to trade the release of his brothers for Chang agreeing to withdraw an article he wrote in Deutsche Welle.

“I am concerned for their safety,” he wrote in the email. “They might be prosecuted for other things framed to them.”

Chang, who lives in Germany, wrote in Chinese about journalist Jia Jia’s detention for Deutsche Welle, before the journalist was released from custody on Friday.

Chang was also interviewed by Radio France Internationale about what he sees as an ongoing power struggle within the Chinese Communist party.

“Following my article and interview, my direct family members and numerous relatives in China have been subject to investigation, harassment, and threats,” he wrote on Sunday, declaring that his family members have no connection to his writings abroad.

Rights groups say authorities in China have recently rounded up more than 20 people to pressure them into disclosing information about the origins of the Xi letter, or to put pressure on exiles abroad to do so.

Last week, New York-based blogger and government critic Wen Yunchao said police in Guangdong province, southern China, took away his elderly father and mother and younger brother on 22 March. In an email on Monday, he said he still had not heard from his family.

The publication of the letter calling for Xi’s resignations appears to have sparked a wide-ranging probe into who might have been involved in its authorship or distribution. The letter appeared right before the start of China’s legislative session, a sensitive time for the Communist party, on Wujie News, a Chinese website. Authorities have since detained at least 16 people who worked for the site. They also detained Jia, who was thought to have warned Wuje News to take the letter off its site.

Before becoming an exile, Chang was chief commentator and news director of Southern Weekend, a newspaper in South China known for its aggressive investigative reporting and columns. In early 2011, he was forced to leave the newspaper. He worked briefly for a now-suspended news weekly in Hong Kong before being denied a work permit, which prompted him to move to Germany.