It was a case of the $2 hongbao.
During the Chinese New Year festive period in the 1970s, postmen would collect red packets from shopkeepers. Those that gave would get their letters delivered intact and on time.
"Those that didn't would have their letters thrown away by the postmen,'' said Mr Raymond Ng, 74, who was then a young investigating officer with the Corrupt Practices Investigation Bureau (CPIB).
It had earlier received complaints that postmen were getting red packets as bribes from shopkeepers.
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Recounting the case of a postman he caught, Mr Ng said he had received a $2 hongbao from a shopkeeper in Joo Chiat Place.
When he was arrested, he pleaded for a second chance. "He went down on bended knees to ask me to let him go... but I couldn't. I had to take him in." The case left an indelible mark on Mr Ng as it underscored Singapore's no-nonsense stance towards corruption.
He was among several past and present CPIB officers who spoke to the media last week about their experiences in investigating graft cases, ahead of the official opening of the Corruption Reporting and Heritage Centre in Whitley Road.
Another memorable case in his 27 years at the bureau involved a senior public servant receiving bribes totalling about $13.85 million.
It was Singapore's biggest graft case, and Mr Ng was on the team that dug out the evidence that convicted Choy Hon Tim, then deputy chief executive of the Public Utilities Board. He was jailed for 14 years in November 1995.
In his early years with the CPIB, Mr Ng said, corruption was particularly widespread among junior civil servants and police officers.
"Although the bureau was set up and corruption was under control, there was a lot of petty corruption," he said.
Today, while corruption cases are at an all-time low, criminals are exploiting technology and the ease of travel to avoid getting detected, said CPIB's head of operations management Thomas Cheo, 45.
Unlike in the past, when bribes were paid locally, nowadays, they could be paid overseas, or with electronic currencies such as bitcoin.
Investigators often have to travel abroad to get evidence like foreign bank records, Mr Cheo said. To be good at their job, "investigators need an inquisitive mind, be good listeners and have an eye for detail".