Catching the Claremont killer | Meeting with a monster

Last week we reported on the controversial West Australian police investigation that has spent millions over the past two decades trying to catch the Claremont serial killer.

The probe into the Claremont serial killings is the longest active criminal investigation in the country. It is also the most expensive in Australian history.

• 'Why we didn't catch the Claremont killer': The botched investigation

• The prove that has haunted investigators for nearly two decades

But almost two decades on, the murders that scarred the city remain unsolved. Now, Paul Ferguson has come out of retirement to talk to a killer.

"I know that the offender thinks at this stage that he or she is smart and they've got away with it," he says.

Catching the Claremont killer: Part two

Does he still believe the killer will be caught?

"Yes, I do."

Mr Ferguson, the original head of the Macro taskforce, says he believes that the killer will be reading this story. So, too, will a work colleague or friend who knows what happened.

"If someone knows something, put yourself in the Spiers family's position, put yourself in the Rimmers' position, put yourself in the Glennons' position . . . think of what they've been through," he said. "He or they have the propensity to do more next time and it might be your sister or your daughter."

Macro was set up in 1996 to hunt for the killer amid extraordinary public and political pressure.

Experts were flown in from overseas, video evidence sent to NASA, and taxi drivers across Perth were DNA tested amid fears that the only car the three girls would have trusted enough to get into would have been a taxi.

At one point, police even sought the help of one serial killer to find another. In an extraordinary revelation, Mr Ferguson now reveals that shortly after Jane and Ciara's bodies were found, he got a message from David Birnie in Casuarina Prison.

"He said, 'Paul, you know if you want to come down and interview me in relation to what's happening over there I can tell you from a serial killer's perspective what I'd be thinking if it was me'," Mr Ferguson recalls.

Ten years earlier, the detective had been part of the team that caught David and Catherine Birnie, a case he still describes as "the most interesting and horrific I've had in my career".

As he talks about it, he points to the back of his head and closes his eyes: "I've got things tucked away back here that I pray to God I never pull out of the drawer."

From prison, Birnie had reached out and suggested to him that the best way to get into the mind of one serial killer was through the mind of another.

"I went down there. I let David do the talking," he says. "It's actually quite strange that there are consistencies in a lot of serial killers.

"The way they do things like dump sites. David confirmed that his 'prowling patch' was Stirling Highway."

The information was relayed back to the Macro taskforce as they tried to piece together the psyche of the Claremont killer.

The WA Police have refused repeated requests to talk about the Claremont case, saying only that it is continuing inquiries.

Mr Ferguson says he has come out of retirement now because the police had refused to speak and he believed the time was right for someone to come forward.

"What I can tell you is that the more people involved in the crime, the more chances of getting caught," he says.

"Relationships change. Friendships break down. Circumstances change. If someone has some information that can lift one person above the others, ring Crime Stoppers."