Bradley Robert Edwards must be found innocent of the Claremont serial killings if the judge does not accept crucial DNA evidence in the case, his defence team says.
The former Telstra technician 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 lengthy judge-alone Supreme Court of Western Australia trial.
Prosecutor Carmel Barbagallo relies on evidence from more than 200 witnesses, as well as fibre and DNA analysis - the most crucial exhibit being Edwards' DNA recovered from a combined sample of two of Ms Glennon's fingernails.
Defence counsel Paul Yovich said the burden rested with the prosecution to explain how "a fraction of a nanogram" of Edwards' DNA was recovered from the sample.
He said if Justice Stephen Hall was not satisfied Edwards' DNA got there during a fight before Ms Glennon's death, then he could not find Edwards guilty of any charges.
Mr Yovich went through a series of contamination events at PathWest, including one incident in which DNA from an unrelated case contaminated an exhibit from Ms Rimmer's crime scene.
"Contamination events in the laboratory are rare, they're even very rare, but they do happen," he said.
Mr Yovich noted there was no DNA from the victims on any exhibit associated with Edwards.
Earlier, he told Justice Hall that elements of the case that do not fit the prosecution's argument must not be glossed over.
Edwards has admitted to violent crimes against three other women before the killings, including twice raping a teenager he abducted from a dark Claremont park and dragged through a nearby cemetery.
Mr Yovich is expected to complete his closing address next week.
Justice Hall could then take months to deliver a judgment on whether Edwards is guilty.