The parents of a woman murdered by her boyfriend five years after he killed another partner say they are "finally being taken seriously".
An inquiry found Sussex Police "missed opportunities" when investigating the deaths of Susan Nicholson, 52, in 2011 and Caroline Devlin, 35, in 2006.
Robert Trigg, 52, was convicted last year of killing them both.
The findings have now been referred to the Independent Office for Police Conduct (IOPC), the force said.
The full report by Thames Valley Police is not expected to be published until other independent inquiries have concluded.
Trigg was in a relationship with both women when he killed them in their homes, which were barely two miles apart in Worthing, West Sussex.
Both deaths were treated by Sussex Police as not suspicious at the time after Trigg claimed they had died in their sleep.
In 2014 Ms Nicholson's parents hired a barrister and a forensic pathologist, Dr Nathaniel Carey, to re-examine the original pathologist's report.
He concluded Ms Nicholson was suffocated by having her head forced into a bed. Trigg had claimed he had rolled on top of her while they pair were asleep on a sofa.
In Ms Devlin's case, he found her death was was caused by a blow to the back of her head, and not an aneurism, as first recorded.
During the trial the court heard both women had suffered domestic violence at the hands of Trigg during their relationships with him.
Ms Nicholson's parents, Elizabeth and Peter Skelton, spent six years and more than £10,000 trying to convince the police to reopen their investigation into their daughter's killing.
They claimed they had been "ignored" and "failed" by police and every public body tasked with holding the force to account.
Mrs Skelton, 81, said: "Finally, we feel like we are being taken seriously.
"The more you look back at it all, the more furious you feel about how it was handled."
Mr Skelton, 83, said: "This confirms what we knew all along. They can't just say this was a mistake.
"We have been going on about this for six years. The evidence was pointing to this the whole time."
At least two police officers were involved in the investigations into both killings, but the similarities between the cases were not regarded as suspicious. In both cases police treated Trigg like a bereaved partner rather than a suspect.
Sussex Police investigated its own officers three times but found nothing wrong with their handling of Ms Nicholson's murder.
A spokesman for the force said: "Thames Valley Police have completed an independent review of the investigations. The review refers to potential missed opportunities and we have therefore referred it to the IOPC.
"We are truly sorry it took so long to get justice, and it is important we learn any lessons and provide answers for the families."
The chairman of a separate inquiry, set up in October and led by the Safer West Sussex Partnership, will examine which services came into contact with Trigg and his victims and what they did to "manage any risks identified".
A draft report is expected in the summer but is not likely to be published until the end of the year.