The arrest of three investigative journalists in Gansu ( 甘肅 ) province has escalated into a nationwide controversy, prompting an investigation by provincial prosecutors and calls from state-run media for transparency from the local police department. The case has gained attention as the trio, stationed in the city of Wuwei ( 武威 ), had all written about the city in unflattering terms, with some of their articles drawing nationwide attention. Such articles had damaged relations between the three journalists and the city’s propaganda officials long before the arrests, a source told the South China Morning Post . Questions over the arrests first arose when it emerged that one of the trio, Zhang Yongsheng, had been detained on January 9 on suspicion of paying for prostitution, which is not a criminal act. READ MORE: Whistle-blowing Chinese journalist vows to continue exposing corruption as Beijing prosecutors drop charges of defamation The details of Zhang’s detention were outlined in a letter sent by his employer the Lanzhou Morning Post to Wuwei authorities in an unsuccessful attempt to seek his release, the source said. The letter subsequently leaked onto the internet and went viral. Wuwei’s police force then said that Zhang Yongsheng, Luo Huansu of the Lanzhou Evening News , and Zhang Zhenguo of the Western Business Daily , had been detained on another occasion – January 7 – for blackmail. All three were formally arrested last Monday, but Luo and Zhang Zhenguo were released on bail, according to the Liangzhou district government’s website. It claimed the trio had often blackmailed people into giving them money by claiming to provide “media supervision”. However, even state-media appears to have doubts about the arrests, and has urged city authorities to be more transparent. “Only more truth and less imagination can calm the public,” CCTV said on Sunday. Those comments came a day after the province’s top prosecutor said it had stepped in to investigate the cases. “There have long been tensions between the city’s propaganda officials and the three journalists,” the source said, adding in 2014 a propaganda official had said openly that he wanted to “kick [the trio] out”. READ MORE: Jailed and refused bail, Chinese investigative journalist detained after exposing high profile corruption scandal The source attributed the tensions to a series of investigative articles. An article by Zhang Yongsheng in 2014 revealed that around a dozen pupils and students in the city had been coerced by gangs into selling blood. The article had prompted follow up coverage by various news organisations, including CCTV. Another article by him in 2011 revealed a Wuwei police officer had helped a fugitive and suspected murderer change his legal name. Shortly before the arrests, Zhang Yongsheng told colleagues he had received threats from local police and been warned against publishing some stories, which he filed anyway, said the source. On January 7, a fire drill in Wuwei had spiralled out of control, according to local news outlets. Zhang Yongsheng had been told by a local propaganda official not to cover the fire, before he became unreachable, said the letter by the Lanzhou Morning Post . About a week into Zhang’s detention, Wuwei police officers visited his paper’s office, asking about his articles, including the blood selling and fugitive stories. All three newspapers have suspended operations in Wuwei, because the trio were their only reporters there, said the source.