They are tiny scraps cut from the fingernails of a murdered lawyer that have been stored in a freezer for more than two decades — and they are at the heart of the Claremont serial killings case.

Ciara Glennon is one of three young women who went out with friends at night in the western Perth suburb of Claremont in 1996 and 1997 and never came home again.

It is the prosecution's case that they were murdered by Telstra technician Bradley Robert Edwards, who allegedly roamed the streets of the wealthy suburb looking for his victims.

The bodies of Ms Glennon and Jane Rimmer were discovered weeks after their disappearances, but teenage receptionist Sarah Spiers has never been found.

Ms Glennon's body was found in bushland on Perth's northern fringe 19 days after she vanished, on April 3, 1997.

Forensic police examine the bush crime scene where Ciara Glennon's body was found. ( ABC News )

Childcare assistant Ms Rimmer was also found in bushland, this time south of the city, on August 3, 1996 — a full 55 days after she was last seen.

The key piece of evidence

The passage of time wrought significant destruction on the women's bodies, especially given they were left out in the elements, limiting the amount of evidence that could be recovered from them.

Of all the samples taken from the two bodies, it is just two minute samples of Ms Glennon's fingernails — one of which was initially labelled "debris only, not suitable for analysis" — on which the case against Edwards largely rests.

In her opening address at the marathon trial, which is expected to last at least another four months, prosecutor Carmel Barbagallo SC outlined why those fingernail snippets were so important.

Carmel Barbagallo is leading the prosecution against Bradley Edwards. ( ABC News: Charlotte Hamlyn )

Edwards, she said, was able to "lure" Ms Glennon into his car or she "was abducted by him in a blitz attack".

"He went on to subdue and incapacitate her and, the state says, to murder her in bushland in Eglinton in the early hours of that morning," Ms Barbagallo said.

She characterised the attack as one of "ruthless efficiency".

But there is significant evidence that Ms Glennon fought for her life, the prosecution says, in a desperate struggle that was to lead investigators to Edwards.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 59 seconds 3 m 59 s A timeline of the Claremont serial killings

Did a fight for her life lead police to her killer?

As well as significant neck and head injuries, Ms Glennon suffered what has been characterised in court as classic self-defence injuries to her right forearm.

These were deep wounds that cut to the bone, likely to have been inflicted with a knife that forensic pathologist Clive Cooke said indicated she had her hands raised in a defensive position, trying to ward off her attacker.

During that ultimately futile fight for her survival, Ms Glennon was "scratching and clawing" at Edwards, Ms Barbagallo argued, tearing off her left thumbnail in the process and damaging the nail on the ring finger of her right hand.

This was particularly noticeable, she said, because of the "beautifully manicured" state of the rest of her nails.

Crucially, what was captured "on or under or around" her nails was parts of her killer's DNA, Ms Barbagallo said.

Forensic evidence subject to intense scrutiny

How the fingernail samples came to be taken, how they were stored for nearly two decades, and how the DNA was extracted and analysed by forensic laboratories in Australia and overseas has been the subject of intense questioning of witnesses over the past fortnight in the WA Supreme Court, where Edwards is on trial for the three murders.

The trial has heard how the fingernails were first cut in the state mortuary during Ms Glennon's post-mortem examination, how they were labelled and stored in the freezer at PathWest, the state pathology laboratory, and how sub-samples were extracted from the nails for testing.

Details of what the PathWest scientists were wearing as they worked on tiny fragments of nail, the brand of cleaner they used to scrub down the laboratory benches before beginning work, and the names and results of the highly specialised DNA extraction tests they performed were all detailed.

Witnesses have been asked to pore over forms, case files and other paperwork relating to the tests undertaken on the samples, what was discovered and what notations on the files were made, by whom, when and why.

The level of detail has been at times mind-numbingly banal and yet, at the same time, absolutely essential for the triple-murder trial because in the end so much rests on two samples — AJM 40 and 42.

Why two fingernail fragments are so important

These little pieces of fingernail from Ms Glennon's left thumb and left middle finger are the only places where traces of Edwards's DNA were found on either the body of Ms Glennon or that of Ms Rimmer.

There is no other genetic material linking Edwards to the murders.

Edwards himself does not contest the fact that the DNA is his, but he does contest how it got there, which is why there has been so much emphasis on the treatment of the fingernail samples.

It wasn't until more than a decade after her murder that male DNA was detected on the nails, after a number of samples relating to Ms Glennon — including AJM 40 and 42 — were sent to the UK for advanced DNA testing not available in Australia at the time.

The two critical samples had been stored at PathWest in sterile, yellow-topped plastic containers ever since they were clipped from Ms Glennon's body as it lay in the state mortuary on April 4, 1997.

Until it was sent to the UK, AJM 42 had only been opened twice.

Extracts were taken in 1997 and subjected to a barrage of DNA tests at PathWest, and the little container was opened again in 2004 when its contents were transferred to a plastic tube and sent to New Zealand.

AJM 40, in contrast, was not opened at all until it went to the UK and was "pristine", according to Ms Barbagallo, having been marked "not suitable for analysis".

Chinks in the DNA argument

Five PathWest scientists have so far taken the stand at the trial, as well as a senior scientist from the New Zealand laboratory, and all have been grilled about the measures they took to minimise the chance of contamination while handing the samples.

Even under the pressure of cross-examination, the scientists have been precise and measured when answering questions, at pains to explain the meticulous way they went about their work on the case.

But defence lawyer Paul Yovich SC has found some chinks in their armour.

Defence lawyer Paul Yovich is looking to exploit weaknesses in the DNA evidence. ( ABC News: Hugh Sando )

This week it was revealed that four control samples — part of a batch sent by PathWest to a laboratory in New Zealand — were found to contain DNA when they shouldn't have.

While forensic biologist Sally Ann Harbison testified this had not affected the AJM 42 sample in any way, the revelation served to cast doubt on PathWest's processes.

Mr Yovich revealed Macro Task Force detectives investigating the Claremont murders had asked senior PathWest scientist Martin Blooms be taken off the case after he wrongly told them a DNA test had been conducted on Ms Rimmer's watch when it had not.

The incident also had no apparent material effect on the case — the prosecution accused Mr Yovich of "muckraking" — but it alluded to the integrity of PathWest, its staff and its procedures.

Mr Yovich's aim in the trial is to cast enough reasonable doubt on the prosecution's evidence that his client cannot safely be convicted.

Whether he succeeds will not be known for many months.