Conviction: How police caught Jill Meagher's killer

They wear dark suits and carry bound folders they use to record observations, and if they talk to the waiting media the remarks will be well-practised cliches designed to provide a reassuring sound bite without hinting at the likely direction of the investigation.

Inevitably they will talk of the "brutal" murder, the "devastated" family, the need to keep an open mind and how they are waiting on forensic tests.

If in the weeks that follow they provide a greater insight, it will be to flush out witnesses in what will be a protracted investigation. If they silently bunker down, it is a fair bet they are expecting an early arrest. For the rule is, the more homicide detectives says publicly the less they know privately.

When they have success in a tough one there will be a post-conviction celebration, usually an investigators' lunch at an inner-suburban pub. Which means the inner workings of a homicide investigation remain secret detectives' business.

And so when award-winning documentary producer Terry Carlyon approached police more than 20 months ago to delve inside the investigation into the 2012 murder of ABC staffer Jill Meagher by serial rapist Adrian Bayley as she walked home from a Brunswick pub, there were concerns.

Eventually police agreed to co-operate with Carlyon on his latest project, Conviction, but it was by no means a unanimous decision. Some felt telling the story might give away investigative secrets that would assist future suspects but the majority view was, as Jill's case created such angst in the community, the full story should be told.

Terry enlisted me as the interviewer and narrator, and what became clear to us very quickly was that the police involved wanted to tell their story.

No longer did they speak in sound grabs and cliches. No longer were they just stern-faced men in dark suits. On camera we see their emotional and physical commitment and learn of the long-term personal consequences.

As veteran investigator Detective Senior Sergeant Ron Iddles says, "Some people talk about that job as like throwing a hand grenade into the crew".

We learnt that while homicide detectives are all different, they have one thing in common – a great sense of empathy.

They are the only ones who immerse themselves in the case from beginning to end – culminating, they hope, with a Supreme Court conviction. And they are the ones who bond with the victim's family, who see these detectives as their only hope for justice.

Jill had been for Friday night drinks with friends and was walking the few hundred metres home when she was approached and followed along Sydney Road by Bayley, who then raped and murdered her in a nearby laneway.

Bayley should have been in jail, as his parole should have been revoked when he pleaded guilty to a serious assault in Geelong. Inexplicably he was freed when he appealed against the severity of sentence.

The Meagher murder shocked, fascinated and outraged a large section of the community as few other crimes have done. So why were so many strangers so moved by this particular murder?

First, while we would like to think all lives are equal in our eyes, they are not. Jill was young, pretty, smart, had a good job and a nice husband, and so many saw her life as more important and her death more tragic.

Second, it was a mystery as initially she was a missing person with no one, other than Bayley, knowing what happened after she left the pub.

And finally she was a "blameless" victim, grabbed randomly as she was, just walking down the street. If it happened to her, then it could happen to us.

The initial suspected missing persons case quickly turned into a probable murder investigation under the command of Acting Detective Senior Sergeant Dave Butler of Homicide Crew Four.

It took just six days from the time of Jill's disappearance to her killer being charged (by fluke the documentary airs on the fourth anniversary of Bayley's arrest), but the impact on police was profound with two members of the forensic team so affected by post-traumatic stress they have not returned to work.

From the moment the case went public the interest was extraordinary, with widespread speculation via social media.

As the then head of the homicide squad, now retired inspector, John Potter, told Conviction: "There was no doubt that the teams were under pressure. This wasn't any other job."

Butler looked at the circumstances and quickly concluded the chances of finding Jill alive were slim and immediately (and logically) concentrated on her husband, Tom.

Many on social media were quick to blame Tom, for if it was him then it was not the type of random crime that left other women at risk.

It is a brutal business treating a distressed and confused man, who is coming to the awful realisation he has lost his wife, as a possible murderer.

Butler recalls: "When she didn't appear on the Monday, it really crystallised for me and I formed the strong view that she was probably dead.

"We were able to eliminate Tom reasonably quickly in this case. And the important thing with that was, let's say we never solved the case, the important thing with that is we can say 'Listen, it's not him, we've proven beyond any doubt that it's not him'."

Tom understood the reasoning and was full of praise for the work of the homicide squad, but in some ways Butler remains more bruised by the questioning.

"It was necessary to do but when you think about it down the track, you're left thinking 'Jeez, we were pretty awful, for the way we've treated this poor guy. Not only has he lost his wife but now he's been treated pretty badly by us in some respects'," he says.

We are taken step by step on how Bayley, a convicted violent sex offender, moved from a name in a file, to a person of interest, to the main suspect, and finally the convicted offender.

With no body, no murder weapon and no witnesses the interview with Bayley would be crucial, and while detectives would have liked more time to plan the questioning they were forced to move because of concerns he could destroy evidence or strike again.

The homicide interview has moved a long way from third-degree intimidation and good cop/bad cop tactics. Now it is a conversation based on persuasion where the suspect is led to the conclusion that telling the truth is the only option.

In this case, the quietly spoken and deep-thinking Acting Detective Sergeant Paul Rowe was chosen to question Bayley, a hard, arrogant veteran of police interviews who would not give ground easily.

Crowded in the adjoining monitoring room were senior police desperate for a breakthrough, with everyone knowing this would be a long night.

Sitting in a caravan more than 100 kilometres away connected by phone was the head of crew, Ron Iddles, who was on several days' leave. He was confident Rowe was the man for the job.

Rowe says of Bayley, "He's a very confident person, to the extent that he was quite relaxed initially. I think he was fairly comfortable in the fact that he thought he had gotten away with it and that he would get away with it".

Bayley gave a fabricated version of his movements that night, then slowly Rowe introduced evidence that dismantled, brick by brick, Bayley's house of lies.

"Once he realised that well, perhaps there was some evidence that we had that he wasn't giving me, he certainly became uncomfortable. He became rattled; at one point in time he became angry to the point that they sent another police member to stand outside the door."

While the interview continued, police were searching Bayley's home where they found a damaged phone SIM card. Butler says when Rowe was told of the potentially incriminating evidence he said, "If there's a God, please let it be Jill's".

After a tactical intelligence officer had a Vodafone technician check the serial number, Butler says she "burst in through the [monitoring room] door, and screamed out; 'It's Jill's!' "

Acting Senior Sergeant Sharon Darcy remembers the breakthrough moment: "The elation in the room, just everyone was so pleased that we've got this bastard. That was the point where we knew that, yep 100 per cent, he's our man."

Eventually the arrogant Bayley cracked and sobbed, not for his murderous past but his bleak future. He gave a self-serving confession, then took police to Gisborne where they recovered the body. Faced with the overwhelming evidence he pleaded guilty, saving her relatives the trauma of a trial.

Butler says, "We were able to reunite Gillian with the family to give them some closure and allow them to bury her".

And Butler has no doubt what would have happened if they had not found the killer. "In my view had we not caught Adrian Bayley, he wouldn't have stopped and we would be dealing with a serial killer."

Conviction: ABC TV, 8.30pm, Tuesday September 27.