Detective Chief Inspector Rob Griffin from East Midlands Major Crime Unit says Susan and Christopher Edwards had taken around £300,000 from her parents.
The couple were today convicted of murdering William and Patricia Wycherley.
They are due to be sentenced on Monday.
After killing "reclusive" William and Patricia Wycherley at Nottingham Crown Court, Susan and Christopher Edwards swindled £245,000 over the next 15 years by pretending they were alive.
They continued to collect their benefits and pension payments, and sold their home with the bodies buried under the back garden, in August 2005.
The couple,who had been in "severe financial difficulties" for much of their relationship, were more than £160,000 in debt when they were arrested last October.
Despite their debts, they spent thousands on film memorabilia including signed photos and autographs of Gary Cooper and Frank Sinatra.
Susan and Christopher Edwards were told by the judge, Justice Thirlwall, to expect life sentences.
Sentencing will take place next Monday.
During the trial, Susan Edwards admitted the manslaughter of her mother but claimed she had been provoked. She claimed her mother had shot her father while she was staying with them, and during an argument afterwards said she shot her mother.
Christopher Edwards, who held a firearms certificate, told the court his wife only told him what she had done after they travelled to Mansfield by coach a week later.
He claimed they buried the bodies early on a Sunday morning after watching the Eurovision Song Contest on TV on May 9.
But prosecutors said the Wycherleys were gunned down for their money and buried the weekend before by both defendants.
The jury in the trial of Susan and Christopher Edwards took just six hours to convict them of murder.
Susan and Christopher Edwards had 15 years to come up with an account of what happen that night in May 1998. In their interviews they didn’t stray from a well rehearsed script, and even used the same words and turns of phrases. I wonder whether they had started to believe their own lies. The Edwards’ acted with cold calculation and meticulousness, killing and burying in an unmarked grave to be forgotten the people who raised Susan. They then spent the next 15 years exploiting and profiting from it."
We didn’t know William Geoffrey, our mother’s youngest brother, at all well. We have built a picture of him in our minds based on what she’d told us and photographs we had seen. Sadly, mum died in 2009. Had she been alive now, she would have been horrified by these brutal murders, and the callous treatment of the bodies afterwards. It would have been extremely upsetting for her to discover the deception and lies practised by her niece — our cousin — Susan and her husband Christopher against her and others for so long, for their own selfish greed and gain..
We would like to thank everyone who helped to bring them to justice. In particular, we’d like to acknowledge the police, prosecution barristers, and the jury for their determination and hard work to get to the truth. Also, to the forensic and gun experts whose expertise helped to destroy their web of lies. We are private people and have found it difficult to have our family thrust into the public eye in such a way..
We have felt powerless throughout this terrible ordeal but, while we cannot change what has happened, there is one thing we can do — we can officially acknowledge William and Patricia’s deaths. Our main priority now is to see that they are finally laid to rest with dignity.”
Detective Chief Inspector Rob Griffin, from the East Midlands Major Crime Unit, has told ITV News Central Susan and Christopher Edwards displayed a heartless attitude from the moment they were arrested.
Susan Edwards sent Christmas cards to relatives of her parents in order to convince people they were still alive.
The cards described how the couple were enjoying travelling around Ireland, when in fact they have been shot dead and buried under the garden of their home.
Susan Edwards, 56, and her husband Christopher, 56, were found guilty of murdering Patricia and William Wycherley at Nottingham Crown Court.
The bodies of the Wycherleys were unearthed from the back garden of their home in Mansfield, Nottinghamshire, last October.
Susan and Christopher Edwards have been told they will each receive a life sentence for the murder of Susan's parents.
ITV News Reporter Ben Chapman is in court.
Judge tells Susan & Christopher Edwards they each receive a life sentence. Their minimum term will be decided on Monday pm. @itvnews
In all Susan & Christopher Edwards stole nearly £250,000 from their dead victims to pay off their debts and fund their lifestyle. @itvnews
Susan and Christopher Edwards have been convicted of murdering her parents. The bodies of William and Patricia Wycherley were discovered in the back garden of a house on Blenheim Close at Forest Town in Mansfield last October.
They were murdered 15 years before, in May 1998. Susan Edwards had admitted manslaughter but denied murder. More to follow.