By Sui-Lee Wee

BEIJING, Sept 17 (Reuters) - Six Communist Party officials will stand trial in China on Tuesday on charges of torturing a man to death during an internal investigation in a case that has exposed the secret workings of the party's judicial system.

Yu Qiyi, 42, a chief engineer for a state-owned investment firm in the eastern city of Wenzhou, drowned in April after being dunked repeatedly in a bucket of ice-cold water, state media said last week. His case sheds light on the detention of party officials under a system lawyers say is rife with abuse.

The officials - five from the party's corruption watchdog, the Central Commission for Discipline Inspection, and a local prosecutor, were indicted on charges of intentional infliction of harm leading to death for the alleged abuse of Yu. They face the death penalty.

Yu had spent 38 days under "shuanggui", a form of extra-legal detention imposed on party officials being investigated for disciplinary violations.

"I believe that these egregious cases in which officials who have been under 'shuanggui' have died rarely enter the judicial process," said Pu Zhiqiang, a lawyer representing Yu's family.

Reuters was not immediately able to locate lawyers for the six accused. Neither the government nor the party has publicly given an account of what happened. The story has received limited coverage in state-run media.

It is not precisely clear why Yu was being investigated, according to his family's lawyers. Si Weijiang, a second lawyer for the family, said it was possibly because of improprieties related to a land deal.

Ousted Chinese politician Bo Xilai was subject to "shuanggui" for 17 months before facing trial last month on charges of corruption and abuse of power.

During his trial, Bo recanted an earlier confession to party investigators saying it was made under psychological pressure. A verdict is expected in the coming weeks.

"We think this (Yu's case) is a real tool to measure whether China wants to become a country ruled by law," said Si.

"That is: 'Should shuanggui be included in the legal system?'"

DUNKED REPEATEDLY

Investigators put Yu's head in a bucket of ice-cold water and held it down repeatedly, eventually causing his death, according to Si. Yu was also beaten and his body scarred by what appeared to be cigarette butt marks.

Yu's death comes as China wages war on corruption. President Xi Jinping has pledged to go after "tigers" and "flies" in the battle against graft, referring to both political heavyweights and low-ranked officials.

Pu said Yu, who was made to strip naked before interrogators dunked his head, struggled and shouted during the questioning session, citing the indictment.

Photographs of Yu taken by family members after his death showed an emaciated man who was "black and blue", Pu said.

Si said it was likely the hearing would be closed to the public, saying authorities had worked to keep information about the trial under wraps such as preventing lawyers from photocopying evidence and restricting the number of lawyers the family could hire.

The question of who authorised the harsh treatment of Yu remains largely unanswered, Pu said, adding the six officials were carrying out orders from the disciplinary commission.

"Who approved the use of such torture? The bucket and the ice cold water were already prepared in that place in advance," Pu said. "These people have not been investigated yet."

Most Chinese criminal trials are over within days, often after just a single day. The country's courts rarely acquit defendants in criminal cases.

Yu's ex-wife, Wu Qian, said she was not confident justice would be done.

"We've noticed a certain degree of interference in the legal process already in preparing for this case," she told Reuters, without elaborating. (Additional reporting by Ben Blanchard; Editing by Dean Yates)

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