(Bloomberg) -- Three wildlife conservation activists were detained this week by local police in China’s northwestern Ningxia region, according to the authorities.

Li Genshan, Zhang Baoqi and Niu Haibo were detained for “picking quarrels and provoking troubles” as well as criminal activities, according to an announcement released Thursday by the Public Security Bureau of Zhongwei city in Ningxia, citing a common charge levied against protesters and activists. On Friday, it said another six people involved in the case have also been detained.

Li is well-known for drawing attention to pollution in the Tengger Desert, the fourth-largest in China and home to one of the country’s most important nature reserves.

Last November, his advocacy helped push the government to send a team of investigators to the site, where they discovered that about 130,000 metric tons of pollutants from a bankrupted paper company had been released into the area. Eleven people, including nine from the paper company and two from local environmental monitoring departments, were punished.

According to an article published by China’s Conservation Area system, Li had been writing articles calling attention to two wind farm projects that caused damage to a protected area for Mongolian gazelles in Ningxia before he was detained. In its announcement, the Zhongwei Public Security Bureau asked people to help provide evidence against the three activists.

China has in recent years made life more difficult for NGOs and environmentalists, with activists telling Bloomberg News earlier this year that they have to avoid sensitive topics and practice self-censorship so they can keep operating.

Chinese authorities last month dropped a case against Li Sixia, a 57-year-old engineer who was imprisoned for more than 600 days for “picking quarrels” after reporting on pollution in her hometown in northwestern city of Ankang in Shaanxi province, according to Caixin.

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