Rewards worth $500,000 for information leading to convictions of people responsible for some of Canberra's most baffling murders have gone begging, with nobody coming forward to claim them since they were offered two years ago.
In September 2012 ACT Policing and the ACT government offered six-figure rewards for information about the deaths of Kathryn Grosvenor, Frank Campbell, Irma Palasics and Susan Winburn.
A spokeswoman for ACT Policing has said further information was received in relation to the appeal at the time and police followed up on a number of new leads.
However, no arrests had been made.
"Police are still investigating and would welcome any information a member of the public has no matter how small they may think it is," an ACT Policing spokeswoman said.
Rewards of $1 million have been offered by some states, including to solve the mystery of the missing Beaumont children, Jane, 9, Arnna, 7 and Grant, 4, who disappeared from Glenelg on January 26, 1966. Victorian Police made a $1 million offer in February this year for information relating to the 2011 disappearance of 13-year-old Boronia girl Siriyakorn 'Bung' Siriboon.
Professor of criminology at the ANU Roderic Broadhurst said fear and family were the major factors that kept people from talking to police.
"Rewards can be useful but they won't work in every case," he said.
"Sometimes people don't come forward because they are frightened or feel responsible or involved in some way."
Dr Broadhurst said investigators often faced an obstacle known as the 'halo effect' after a person had died.
"It's a well known phenomenon where somebody dies and we become very reluctant to speak ill of the dead," he said.
"It's especially true of parents. They can talk about their 'angel'. Parents are particularly difficult people to work with if they really loved the child because they will cover things up about their children."
Dr Broadhurst said revisiting cold cases was worthwhile for multiple reasons, including reassuring the community that police did not simply give up on murder cases even if decades had past.
"Cold case units have proven very affective in some cases," he said.
"These days they return to a lot of bio-data like DNA that was collected at a time before technology existed to examine it.
"Also the halo effect can wear off over time and it can be useful to go back and ask more questions about the victim and you sometimes get significantly different versions of what the victim was like and how they behaved."
That extra information could help investigators discover who the victim associated with and that information could lead to new suspects, Dr Broadhurst said.
Anybody with information about the deaths of Kathryn Grosvenor, Frank Campbell, Irma Palasics or Susan Winburn is asked to contact Crime Stoppers on 1800 333 000.