The indictment, details of which were widely published in Chinese media, describes how 42-year-old Yu Qiyi drowned after having his head repeated pushed into a bucket of ice water.
Yu was detained on March 1 by agents from the party's corruption watchdog in the eastern province of Zhejiang and died 38 days later after being rushed to a hospital.
While under questioning, he was held in a detention center run by the party's Central Commission for Discipline Inspection, a body that critics say operates without legal constraints and frequently coerces confessions from those under investigation.
"From the start, I'd hoped the case could be resolved under the law and the perpetrators held accountable and severely punished, but sometimes that's hard to do in China," Wu said in a phone interview.
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Wu said at the time of his death Yu was emaciated, with bruises on his arms and thighs, dark welts on his buttocks and scrapes on his feet and shins. While that appeared to indicate that he was starved and beaten, the indictment made no mention of other forms of torture besides dunking.
Wu's Shanghai-based lawyer, Wu Pengbin, did not answer calls today.
Well-known Beijing human rights lawyer Pu Zhiqiang, who has advised on the case, said there was little for Wu's legal team to do but watch the case proceed. He said Yu's death was not an isolated incident, but rather part of an institutionalized culture of torture within the party investigation system.
"They had to have authorization from their supervisors for what they did, and it is not fair for only them to get blamed.