The apparent suicide of a prisoner who died with the word 'assault' scrawled on his hand is being investigated by police. Some 124 inmates have sent letters raising suspicions over the death of Au Kwok-wah to the Correctional Services Department (CSD). Au, 44, was found hanging from torn bed sheets at Shek Pik Prison on Lantau last Friday. The message on his hand was discovered by hospital staff the next day. The police investigation would consider an allegation of assault, the inquiry leader, Senior Inspector (Crime) Dave Mattinson said. 'My main aim at the moment is to establish if it was suicide or not,' he added. Au was 10 years into a life sentence for murdering prostitute Yeung Shuk-yee. The CSD has set up a five-officer team to investigate. It will report to Commissioner Raymond Lai Ming-kee, who is in charge of internal complaints. 'We try our best to prevent prisoners from hanging themselves,' Mr Lai said yesterday, although he admitted there had been clashes between officers and inmates. A Stanley Prison inmate hanged himself on the same day, bringing the prison suicide tally for the past three years to 16. Human rights groups have raised concerns at the CSD's ability to investigate its own system and want an independent body set up. 'If the allegation is true, it might have been prevented by a proper system of independent inspectors or an improved JP visit system,' said Law Yuk-kai of Hong Kong Human Rights Monitor. 'If it is false, then the same would at least help the CSD clear its name.'