A $1 million reward is on offer to help solve a cold case double-murder of a brother and sister killed in Melbourne's south-east more than three decades ago.
Doris McCartney and Ronald Swann were found dead in their Keith Street home in Moorabbin on October 22, 1989.
A friend of Ms McCartney discovered the pair's bodies and contacted police.
Ms McCartney, 71, and Mr Swann, 69, appeared to have been assaulted.
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But despite an "extensive and exhaustive investigation" over the past 31 years, detectives have been unable to establish a clear motive for their murder.
Detective Inspector Tim Day of the Homicide Squad said investigators were determined to solve the case and provide answers to the family even after all this time.
"This is a family that has suffered the loss of two loved ones in the most horrific circumstances, without ever knowing why," he said.
"Over the years we have followed up a number of avenues of enquiry and interviewed several people.
"This is very much an active case – some of those lines of enquiry are still open to us and have only emerged relatively recently.
"We are hopeful this has put us on the right track and investigators still believe we have the ability to solve this case, even after all this time."
Ms McCartney was a widower and her younger brother had moved into her Keith Street home to support her, police said.
There was no sign of forced entry into the pair's home and nothing had appeared to be taken.
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"From all accounts they lived a quiet life and police have never been able to determine why they were both murdered," Detective Inspector Day said.
However enquiries over the years have led police to believe someone came into the property to meet with Ms McCartney and an altercation of some kind unfolded.
A number of people have been interviewed about the double-murder, but no one has ever been charged.
Police have appealed for anyone with information about the deaths to come forward.
A reward of up to $1 million will be paid at the discretion of the Chief Commissioner of Victoria Police for information leading to the arrest and conviction of the person or those responsible for the crime.
"Today is not only about appealing to the public for information but also offering the opportunity for whoever is responsible to come forward and give their account," Detective Inspector Day said.
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"There will be someone out there why knows why Doris and Ronald were killed and by who – 31 years is a long time to carry a secret.
"Alternatively, it might be this reward is exactly the motivation someone needs to speak up.
"There are likely to be people out there with their own theories about what has happened and why, and we are keen to speak with those people.
"I'm asking anyone out there who does have information about Doris and Ronald's death, no matter what that information might be, to please come forward and speak to police.
"It is no less than their family deserves after all this time."
Anyone with information about the murders of Doris McCartney and Ronald Swann is urged to contact Crime Stoppers on 1800 333 000.
