The graft-buster behind an investigation into vote-rigging cases in the 2011 district council election has lamented the probe's failure to identify and convict the mastermind.
With more than 1,600 complaints received by the corruption watchdog, the King's Park constituency of Yau Tsim Mong District became the centre of attention as 48 people were charged and 45 convicted.
But Eddie Chan Yin-chiu, chief investigator of the Independent Commission Against Corruption, said officers did not have enough evidence to bring the mastermind to court.
"Many of the arrested were staff of the same restaurant. Unfortunately, the Department of Justice said there was not enough evidence," he said.
Edward Leung Wai-kuen won the seat by just two votes in that election. But he was recently unseated when the High Court ruled that he had broken election advertisement rules.
Chan said investigators had worked very hard to establish the relationship among those arrested in an effort to prove that they had conspired to vote.
They even approached those were already in prison as well as the lawyers of those who had pleaded guilty, to see if they were willing to become witnesses and to identify the mastermind.
"Unfortunately, until now, no one has been willing to help us," he said, adding that it was difficult to approach some of the potential witnesses "because they dodged us".
Many also exercised their right of silence after they were arrested.
Chan said he felt pressure handling what he described as the biggest vote-rigging case in the commission's history because of the public attention. But he stressed that he did not face any pressure from his superiors.
He also found no evidence that anyone - including the Central Government Liaison Office, as some had suggested - was behind the vote-rigging.
But while Chan could not bring the mastermind to justice, his colleague, investigator Steven Ng Ka-kin, managed to kill two birds with one stone.
Ng sent two prison officers to jail - in inmates' clothes.
Officers Ng Shuk-yi and Lai Sau-wai were sentenced to 18 and 21 months behind bars last year for asking an inmate and her boyfriend for cash and gifts after illicitly relaying messages for them.
When the investigator first got wind of the case, only one officer was suspected to have asked for advantages from jailed mainland woman Wu Qiaomei and her boyfriend Lu Guoxiang.
They first arrested Ng Shuk-yi and Lu on suspicion of bribery in 2010. When forensic officers examined Lu's mobile phone, they found a voice clip accidently recording Lu's conversation with another officer, Lai, which suggested that Lai had also been taking bribes from him.
"It was really like 'buy one get one free'," Steven Ng said.
The two prison officers did not know what the other was doing and had been using fake names to keep in contact with Wu and Lu.
Wu was jailed again when she returned to the mainland. Only Lu appeared in court as an immune witness.
Steven Ng had to rely on traces of evidence investigators carefully identified in vast amount of documents to prove that the fake names actually referred to Ng Shuk-yi and Lai.
"It's quite ironic. It was their responsibility to look after the prisoners. Now they have to wear the same clothes," he said.
The investigator, who received a commendation from the ICAC operations directorate for cracking the case, said there was no evidence to show that taking bribes was prevalent in the Correctional Services Department.