* Six party investigators jailed for torturing official to death
* Testimonies underscore harshness of party's detention system
* Official died after being dunked repeatedly in bucket of ice-cold water
* President Xi Jinping has ordered crackdown on corruption
* China's top graft buster urges "shock and awe" for offenders
As Yu Qiyi's interrogation entered its 39th day, officials from the Chinese Communist Party's anti-corruption watchdog debated how to get a confession out of the detained man, the chief engineer at a state-owned firm in eastern Wenzhou city.
One official noted he had forced Yu's head under water the night before. A day later, Yu died after being dunked repeatedly in a bucket of ice-cold water.
Six officials were convicted last month of torturing Yu to death. Testimony given in the case, seen by Reuters, illustrates the brutality of a secretive detention system for party members and the drive to get confessions as President Xi Jinping presses on with an aggressive anti-corruption campaign.
Lawyers say the case - highly unusual because Yu's interrogators were charged - also renews questions about the legality of the process given rampant abuses in the system.
The party introduced the detention system, called "shuanggui", in 1990 to weed out corrupt members as the temptation to take bribes sky-rocketed on the back of China's nascent economic boom.
Detentions can last indefinitely, with family members often kept in the dark about the fate of their loved ones.
"If there are more corruption investigations, there is greater use of 'shuanggui'," said Nicholas Bequelin of New York-based Human Rights Watch.
"Because of the political premium that is put on the anti-corruption campaign, I assume this will create the incentive for more abuses."
Prominent human rights lawyer Mo Shaoping called "shuanggui" unconstitutional.
"TERRIFIED OF WATER"
Yu, 42, was detained on March 1. It is not precisely clear why he was being investigated. His family's lawyers believe it was possibly related to a land deal.
The court on Sept. 30 sentenced five officials from the party's Central Commission for Discipline Inspection in Wenzhou to between four and 14 years in jail. A sixth official, from the local prosecutor's office, was jailed for eight years.
It was a rare instance of legal punishment handed down over the abuse of a party official detained under "shuanggui", lawyers for Yu's family said. They said the case went to court because of a public outcry over Yu's death as well as the family's doggedness in seeking justice.
One of the convicted officials, Cheng Wenjie, said senior officials from the Central Commission for Discipline Inspection in Wenzhou gave the go-ahead for the water punishment.
"Yu Qiyi was terrified of water," Cheng told investigators in testimony that detailed the debate on how to get Yu to confess.
"If we continue dunking him in water, we might be effective, so the leaders said continue," according to a transcript of his remarks given to the court in the nearby city of Quzhou, where the trial was held.
His testimony and those of others were part of the defence statement given by Chi Susheng, a lawyer for Li Xiang, another of the six accused. Chi posted the testimonies online, where they have gotten little attention.
Wu Qian, Yu's ex-wife, told Reuters she believed Yu was innocent of any graft accusations. Yu, she said, had "thought he was just assisting with the investigation into other people".
She said Yu told her that commission officials had notified him in January 2012 he was being investigated. Wu added she had no contact with him while he was under detention.
The Wenzhou commission did not respond to a request for comment.
BO XILAI HELD FOR 17 MONTHS
Graft oils the wheels of government at almost every level in China, which ranked 80th out of 176 countries and territories on Transparency International's corruption perceptions index, where a higher ranking means a cleaner public sector.
Nearly all senior government personnel as well as top executives at state-run firms are members of the Communist Party.
Like his predecessors, Xi Jinping has warned that corruption threatens the party's very existence.
Spearheading his crackdown on graft is Wang Qishan, head of the Central Commission for Discipline Inspection.
Wang warned party investigators on Wednesday that their jobs were on the line if they failed to root out corruption, telling them to use "shock and awe" on their targets, in comments published on the commission's website. He gave no details on tactics to get results.
Xi himself has vowed to catch high-flying "tigers" as well as low-ranked "flies".
"Shuanggui" is used for both.
Ousted politician Bo Xilai, sentenced to life in jail last month for corruption and abuse of power, was held for 17 months. In court, he recanted an earlier confession to party investigators saying it was made due to psychological pressure.
Two other party officials died while in detention between April and June this year, according to a lawyer involved in those cases who declined to be identified for fear of retribution.
Commission officials in Beijing declined to comment.
Party members suspected of corruption first go into "shuanggui" as opposed to police detention.
Nearly all are forced to confess, said a lawyer who has represented eight clients held under the system.
"This tool has become a knife hanging over the head of every party member," said the lawyer, who declined to be identified as he has been warned by authorities not to speak to foreign media.
"It makes the people below obedient."
Once a confession had been extracted, most cases are handed to the judiciary, added Mo Shaoping, the human rights lawyer.
"WE ARE ALL ANTS"
The brutality against Yu drew public outrage. Photos of his bruised corpse were put on the Internet by his family before the six officials were indicted.
One Chinese netizen posted an online comment calling the abuse of Yu "fascist". Another said: "In the face of strong power, we are all ants, there's no exception even for people within the system."
Even official news agency Xinhua weighed in, saying in an online posting that "if you do not lock power into the system's cage, it will be difficult for anyone to feel a sense of security".
This week, China's prosecutor-general, Cao Jianming, said some investigators relied too much on confessions rather than evidence, although he did not refer to the party's detention system.
The cause of Yu's death was "inhalation of liquids leading to pulmonary dysfunction", according to the defence statement from Chi, the lawyer representing one of the accused.
"When I arrived at the scene, I was very shocked because he had changed dramatically, he was emaciated," said Yu's ex-wife, Wu. "The doctor told me there was no point in resuscitating him, there was no hope."
One official responsible for guarding Yu testified that he beat the detainee on a number of occasions.
"Because he wasn't honest, (I) beat him twice," said Yang Huan. "Each time, (I) slapped him about two to three times. After that, (I) dragged him once to the toilet and slapped him about four to five times."
On March 13, when Yu nodded off to sleep, another guard kicked him, according to testimony from Zhang Yuexiang, a worker assigned to watch over Yu.
ONE DETAINEE RECOUNTS SUICIDE ATTEMPT
Lawyers and scholars familiar with "shuanggui" said they were not aware of any data on the number of party members who had been detained under the system. But they say it's a frequent practice.
"There's definitely a correlation between shuanggui and Xi Jinping's anti-graft campaign because the disciplinary commission will first use shuanggui and then pursue judicial proceedings. It's a useful tool for the government," said human rights lawyer Si Weijiang, who is advising Yu's family.
"There's no time constraints, there are no legal rules to follow, the person in shuanggui has no right to a lawyer."
The former head of a police station in eastern Jiangsu province who was held for 42 days in 2004 told Reuters he tried to commit suicide because of the abuse he suffered.
"They hung a 20-kg sandbag around my neck to try to force me to admit I had taken bribes," he said. "When you experience those kinds of things, you want to commit suicide but they made me wear a helmet so that I couldn't smash my head against the wall in an attempt to die."
The former officer, who declined to be identified for fear of retribution, said he eventually confessed. He said the case was the result of a personal vendetta from a former colleague.
The officer, now 48, said he was sentenced to a year in jail for bribery.
The Central Commission for Discipline Inspection in Jiangsu did not respond to a faxed query from Reuters.
But an interrogator in Chongqing - the southwestern city run by Bo Xilai before his downfall - defended the detention system, saying abuses were not the norm.
"It's just colleagues having a discussion, we don't have any right to come into physical contact with them," said the interrogator, surnamed Wang, who declined to give his full name. "We have basic procedures and measures that we must follow."
In the early hours of April 9, the six officials in Wenzhou panicked when they heard Yu had died in hospital, according to testimony by Cheng Wenjie, one of the convicted officials.
Cheng said Liu Xianfeng, a senior official within the Wenzhou commission, told him and two others to accept responsibility for the case "because the leaders said the fewer people who are involved, the better".
One official who testified to police, Chen Zheyi, said that Liu had told him to delete surveillance footage of Yu's questioning. Liu could not be reached for comment.
When asked by police why he gave instructions to do so, Liu said in testimony that he did not want to involve too many people.
"Whoever I can protect, I should protect," Liu said. "Besides, it's not good for the media to get wind of this."