Graft-busters investigated a team of prison officers but found evidence to prosecute only one case of falsifying the urine samples of released prisoners, a security official told legislators yesterday. Undersecretary for Security John Lee Ka-chiu was responding to a question about a significant drop in the success rate for the rehabilitation of drug-related offenders since the arrest of two officers in 2009. Tang Kwai-man, 49, and Leung Siu-wing, 53, were earlier this year jailed for three months for switching the urine samples of released inmates with their own in what was said to be an effort to boost the apparent rehabilitation rate of offenders. "Is the problem very common? When ICAC handled this case, it thoroughly investigated the whole team. There was only one case prosecuted," Lee said in response to a question from lawmaker Wong Yuk-man. Tang and Leung, who have since appealed against their convictions, claimed that urine-swapping was common practice, ordered by senior officers. At a meeting of the Legco security panel, Wong questioned figures showing the rehabilitation rate of released inmates after the arrests had fallen from 64.7 per cent in 2009 to 49.7 per cent in 2010 and 42.2 per cent in 2011.