The Claremont serial killings trial judge has been urged to reject Bradley Robert Edwards' "desperate" DNA contamination theory and give weight to "extremely strong" scientific support for the proposition he is the murderer.
In her closing submissions in the Western Australian Supreme Court on Monday, prosecutor Carmel Barbagallo told Justice Stephen Hall that DNA evidence comprehensively proved the former Telstra technician was the killer.
The 51-year-old denies murdering solicitor Ciara Glennon, 27, secretary Sarah Spiers, 18, and childcare worker Jane Rimmer, 23, in 1996 and 1997.
But he has confessed to raping a teenager he abducted from a Claremont park and dragged through nearby Karrakatta Cemetery in 1995, and indecently assaulting an 18-year-old woman as she slept in her Huntingdale home in 1988.
The prosecution argues DNA recovered from the rape victim and from a stolen silk kimono left behind at the Huntingdale property links Edwards to scrapings from Ms Glennon's fingernails, some of which broke as she scratched her attacker.
She fought for her life "and in doing so took a piece of the accused with her", Ms Barbagallo said.
The genetic markers on the DNA samples were "unambiguous", she said.
Forensic scientists had concluded they were at least 80 million times more likely to have come from Edwards than an unknown man unrelated to him, which was "extremely strong scientific support" for the state's allegations, Ms Barbagallo said.
The defence team suggests the nail samples may have been contaminated by DNA obtained from the rape victim, but the state says that is implausible.
Ms Barbagallo repeatedly stressed the container for one of two critical scrapings had never been opened before it was eventually sent to a UK laboratory, where genetic material was extracted in 2008.
It had been deemed unsuitable for testing at PathWest in Perth, but "may well have turned out to be the proverbial pot of gold".
The other critical exhibit had only been opened twice before arriving at the UK lab but both occasions were not close "in time or place" to the rape evidence examinations.
It was "unfathomable" Edwards' DNA had hung around the PathWest lab "evading all cleaning regimes".
"The probability of any contamination event was extremely remote," Ms Barbagallo said.
"We say contamination is a desperate theory."
The kimono was only visually examined in 1988, returned to police weeks later and didn't enter PathWest until 2016 when Edwards' DNA was found on it.
Ms Barbagallo foreshadowed the defence team seizing on examples of contamination at PathWest in its closing submissions, but that happened only 10 times over the two decade-long investigation.
Nine were laboratory staff contaminating exhibits with their own DNA, so were irrelevant, she said.
The other one was DNA from the victim of an entirely unrelated case, which was found on vegetation removed from Ms Rimmer's crime scene, but it was not a critical exhibit and how it happened was explicable.
Australian Associated Press