Local officials in China are using secret "black jails" to imprison citizens who threaten to report corruption to central authorities, an Al Jazeera investigation has revealed. At one location Al Jazeera's Melissa Chan found a woman being held by three men. As they slammed the door shut, the woman was heard screaming: "You're here to save me! I'm trying to get out!" Another woman, Shan Yajuan, said that she had previously been held in a "black jail" where she was beaten and shocked with cattle prods.