Three anti-graft officers, including a top investigator, were convicted of perverting justice and misconduct for coaching a key witness to provide false evidence in a HK$100 million warrants fraud case. Kevin Cho Wing-nin, chief investigator of the Independent Commission Against Corruption, became the most senior graft fighter charged and convicted by a court since the ICAC was established in 1974. The ruling is a blow for the ICAC after it boosted its reputation with last month's high-profile arrests of Sun Hung Kai Properties tycoons Thomas Kwok Ping-kwong and Raymond Kwok Ping-luen and former chief secretary Rafael Hui Si-yan. Barrister Stephen Char Shik-ngor, a former ICAC investigator, said: 'There will be some impact on investigators' morale ... the convicted officers were not trying to obtain any personal benefit from this. 'They were just doing their job, but they crossed the line.' An ICAC spokeswoman expressed regret over the case yesterday and said the commission respected the ruling. She said the ICAC would not accept any illegality in its investigations and would strengthen training for its officers in response to the case. Cho, 47, and senior investigator Ben Chan Kai-hung, 39, were found guilty of perverting justice - the first time ICAC officers have been convicted of the offence - and misconduct in public office. Assistant investigator John Au Kim-fung, 42, was convicted of misconduct but acquitted of perverting justice. They were charged with coaching key witness Cheung Ching-ho, 39, in a fraud trial against celebrity warrants trader Raymond Ng Chun-to between November and December 2009. Ng was jailed for four years in April 2010. Cheung was one of five people arrested in May 2008 for manipulating the derivatives market and laundering more than HK$100 million. He had originally been due to give evidence under immunity from prosecution against his co-accused, Ng. Yesterday, District Court judge Douglas Yau Tak-hong said although he sympathised with the convicted officers, he would have to remand them in custody until April 30 for sentencing because of the gravity of the offences. 'It would be unfair to give them false hope. I have no choice but to sentence them to a term of imprisonment,' Yau said. Cho and the other two investigators tried to coach Cheung on how to testify in the trial. Cheung secretly taped conversations on November 30 and December 3, 2009 in which he deliberately prompted the officers to instruct him on how he should testify in court. Earlier, Cheung told the court in a separate trial that he was 'coerced' into co-operating with the ICAC officers and he had taped the conversations after consulting his lawyer. Cheung did not elaborate on why his lawyer made the suggestion. Cheung later reported the case to police, after failing to gain an agreement from the ICAC to drop the charges against him. He is serving a 25-month jail term for his part in the fraud. The judge said yesterday it was 'obvious and deliberate' that Cheung had led the officers into the trap. 'Unfortunately, they could not resist the temptation and wanted more than what Cheung had told them,' he said. But he accepted the authenticity of the secret recordings.