A freed rapist was sent back to jail yesterday for four years after trying to extort HK$6,000 from one of his former prison guards.

Debt collector Tam Chi-kin had sent threatening letters to Lung Sai-fan - who had apparently helped prove he had broken jail rules as an inmate 20 years earlier - prompting Lung to fear for the safety of his family.

Following a trial at the District Court, Tam, 43, was convicted of criminal intimidation, blackmail and money laundering.

Records showed that while Tam was in jail for rape, he was found to have breached rules in 1989 and 1990 for possessing banned items. He remained incarcerated for 10 more days as a result, and missed his father's funeral, the court heard.

Fast forward 20 years to May 2010. Lung, an assistant officer of the Correctional Services Department, received a letter instructing him to call a number. When he did, the man on the other end of the line alluded to Lung having played a part in a negative report about that man while he was jailed in Stanley Prison about two decades earlier. He demanded HK$6,000 from Lung.

Several months later, Lung received another letter, which mentioned the names of his wife and son. He followed police advice and deposited a cheque into an account specified by the man. Police found Tam also used the account, belonging to a woman he knew, to launder HK$35,000.

A former neighbour of Lung's had received phone calls in which the caller claimed she owed HK$130,000, the court heard.

"The blackmail involved a Correctional Services Department officer and not only that, but also his family and neighbours," District Judge Frankie Yiu Fun-che said.