The detained relatives of an outspoken Chinese dissident living abroad committed arson, according to police, after being held in what is widely seen as a crackdown following the publication of a letter condemning President Xi Jinping.

Authorities in Sichuan province said on a verified social media account that German-based journalist Chang Ping's father and two younger brothers were being investigated for causing a forest fire after lighting incense as part of an ancestor worship ceremony.

"The Xichong Public Security Bureau has opened an investigation in accordance with the law into Zhang and his two sons," said the statement posted on Sina Weibo, a Chinese version of Twitter. Zhang, who was only referred to by his surname, was identified as Chang's father.

They are accused of burning down about 40 mu (2.6 hectares, 6.4 acres) of forest in a fire that took roughly an hour to extinguish, the police said.

Writer Chang -- whose given name is Zhang Ping -- had previously said authorities detained his two younger brothers and a younger sister in connection with suspicions that he had been involved in writing an anonymous letter calling on Xi to step down for the good of the country.

Chinese authorities have detained several people in what appears to be a reaction the letter, which was attributed to "Loyal Communist Party Members" and appeared on Wujie News, a state-backed website, before it was deleted.

Those held include a number of staff members at Wujie, who went missing around two weeks ago. Last week, New York-based writer Wen Yunchao said officials in the southern province of Guangdong had taken away three of his family members.

Chang and Wen have both denied any connection to the letter.

Chang, a prominent commentator on contemporary affairs, was formerly a senior journalist at the outspoken Southern Weekend newspaper but moved to Germany after coming under sustained pressure for advocating more government openness and accountability.

The police statement did not mention his sister.

Sichuan police also accused foreign media of "hyping" the detentions, a common government refrain when officials are displeased with media attention.

Media criticism of top leaders is almost unheard of in China, where the press is strictly controlled by the ruling Communist Party.