Bradley Robert Edwards has been "unmasked" as the Claremont serial killer after months of evidence that gradually cast a light on the "enigma of the dark", prosecutors say.
The 51-year-old denies murdering secretary Sarah Spiers, 18, childcare worker Jane Rimmer, 23, and solicitor Ciara Glennon, 27, in 1996 and 1997, but did not testify at his judge-alone WA Supreme Court trial.
Prosecutor Carmel Barbagallo wrapped up her three-and-a-half-day closing address on Thursday, saying there was both physical and circumstantial evidence against Edwards.
"Nearly seven months ago, in this very place, the state made reference to an enigma of the dark and promised to shed light on and demystify that enigma," she said.
"We say that that light has been cast incrementally from the months of many witnesses."
Of more than 200 witnesses, some still had vivid memories while the recollections of others had faded over time.
"But what has remained impervious to the two-decade span is the forensic evidence," she said.
"Physical evidence cannot be intimidated, it can't forget.
"(The evidence) has cast light on and unmasked the killer sought by so many and for so long."
Ms Barbagallo said it was difficult to argue against the logic that one offender was responsible for the murders.
"Bradley Robert Edwards, we say, is the Claremont serial killer."
Edwards admits attacking three other females, including a 17-year-old girl he abducted from a dark Claremont park then raped in a nearby cemetery in 1995.
Prosecutors say the murder victims were either abducted or lured into his Telstra work vehicle, which looked like a taxi, after nights out in the affluent Perth suburb.
All the crimes were allegedly sexually motivated, with Ms Rimmer's body found naked in bushland, while Ms Glennon was discovered in scrub at the opposite end of the city with her skirt up around her waist.
Ms Spiers' whereabouts remains a mystery.
The state relies heavily on fibres it says came from Edwards' work car and clothes that were found on Ms Rimmer and Ms Glennon.
But the crucial physical evidence is DNA extracted from Ms Glennon's fingernails, some of which tore as she fought for her life.
Defence counsel Paul Yovich has suggested contamination is possible.
He said in his closing address that there was "no satisfactory explanation" for a scratch on a container holding Ms Glennon's thumbnail clipping.
It was possible someone opened the container and applied an instrument, he said.
"It's a question of the continuity and integrity of the most crucial exhibit."
Prosecutors say the container remained sealed from the post-mortem until 2008 when the sample was combined with another from Ms Glennon's nails and tested in a UK lab, when Edwards' DNA was recovered.
Justice Stephen Hall said it appeared to be "dried smear marks, dried droplets" rather than scratches, which would be consistent with swabbing done in 2008.
Mr Yovich also zeroed in on the small size of the trace DNA sample.
"The state's case hinges on something in the order of one-fifth of one-billionth of one gram of cellular material," he said.
The lawyer urged against glossing over matters that did not fit the state's case or explaining away inconsistencies.
"We must be careful not to form a view based on certain aspects ... that appear more persuasive."