They are the sliding door ones, where the slightest change of circumstance could have brought about justice rather than open files and unanswered questions.

Peter MacDiarmid sat in the lounge room of the family home with his wife Sheila by his side as he described their daughter Sarah's last known movements nearly four years earlier. For me, it was a relief he was talking in a matter-of-fact way because it made me feel as though I wasn't intruding on their almost unbearable grief.

And then he burst into tears. Sitting next to me was a blonde, female photographer waiting to take a snap for the story. "I'm sorry," he explained to her, "the sun just hit your hair like it would Sarah's."

That is the burden of not knowing for relatives of murder victims. It simply never goes away.

On July 11, 1990, Sarah played tennis with workmates then caught a train to Kananook Station, arriving about 10.20pm. Police believe she walked to her car carrying her backpack, which contained her work clothes, tennis racquet, Walkman and about $60.

The strongest police theory is that a group of homeless drug addicts jumped Sarah as she walked to her 1978 Honda Civic at the railway station car park, stabbed her and stole her backpack. Her body has not been found. Eventually Peter, Sheila and son Alisdair moved to Brisbane to try and escape the daily physical reminders.

Peter found that if his eye caught some dumped rubbish he would stop and search in the hope it would lead to Sarah's missing bag. "You would drive around looking for clues. It hurt too much."

For Roger and Joy Membrey, the sharp pain of intense grief has been replaced by the dull ache of not knowing. Their daughter Elisabeth was murdered in her Ringwood home in December 1994. And like Sarah, her body has not been found.

There were suspects, hope, false dawns and finally, a man charged. Shane Andrew Bond was a regular at the Manhattan Hotel where Elisabeth worked and was implicated by several witnesses as the killer. In 2012 Bond was acquitted of the murder, but has been left damaged by the investigation and trial. He became an ice user and has been moving around the country searching for anonymity.

Naturally, the Membreys want the police to keep looking. And the Membreys have an ally from the strangest corner - one of Bond's relatives is calling for a reinvestigation to catch "the real killer".

Deborah Boundy was killed by a ruthless group of gangsters to stop her implicating them in a paid hit, even though she had risked her own freedom to protect theirs.

Yet after her murder many believed she had jumped bail, with police placing her on the top 10 most wanted list.

When hitman Christopher Dale Flannery and his murder partner Kevin "Weary" Williams were charged with the murder of Melbourne businessman Roger Wilson, Boundy became the star prosecution witness.

Wilson went missing on his way home to Narre Warren on February 1, 1980 and his body remains undiscovered.

Boundy (who was having an affair with Williams) at first implicated the hit team but later retracted.

Flannery, and those close to him, believed Boundy remained a loose end - even though she had refused to implicate them at the inquest. And Chris hated loose ends.

On Christmas Day 1980 she had lunch with her parents, reported for bail (charged with perjury when she retracted her statement at the inquest) and then slipped over to a Richmond pub for drinks.

Someone she knew lured her from the Royal Oak Hotel in Richmond, then forced or tricked her into writing a letter to her parents saying she was running away.

So who was the killer?

The chief investigator in the case, Frank Bellesini, says gangster Alphonse Gangitano (murdered in 1998) was the main suspect. Flannery was acquitted of the Wilson murder but went missing in Sydney in May 1985. Like Wilson, his body was not found.

But the man alleged to have ordered the hit on Wilson and the woman who lured Debbie from the pub remain alive. She wisely keeps a low profile, while he occasionally takes to Twitter as if his past has been forgotten. It hasn't.

Conventional wisdom is that serial sex offenders don't stop until they are caught or incapacitated, which makes the case of Karmein Chan so baffling.

Karmein, 13, was abducted from her Templestowe home more than a quarter of a century ago in what was clearly a well-planned crime.

The offender did his homework and hit the large house surrounded by a two-metre fence when parents, John and Phyllis Chan, were at the family business, a popular Chinese restaurant in Main Road, Lower Plenty.

Occasionally I would eat there with then assistant commissioner Frank Green and his wife, Norma. They were regulars, where Phyllis would entertain front of house as part host and part comedian. Years later Frank's wake was held in the same room.

Most police involved in the case believe it was a child abductor dubbed Mr Cruel, responsible for at least three other attacks from 1987 to 1991.

In the three previous cases the victims had been released, which fed the hope Karmein would eventually be freed. But a year later her body was found at Edgars Creek, Thomastown. She had been shot three times in the head, execution-style.

This didn't fit Mr Cruel's profile, as he had previously assured his victims they were not in danger. One told police: "He appeared to be acting out a fantasy like he was married to me. He showed this by the affection he showed to me and how chummy he was to me."

Mr Cruel was obsessive about removing any possible forensic evidence - leading police to suspect he may have had some law enforcement experience. He also concealed his face from victims. Mr Cruel was constantly in the headlines, and Karmein told friends that placed in similar circumstances she would fight.

This led police to speculate she may have grabbed his mask and was killed because she could identify him.

So why did he stop? Perhaps the killing destroyed his elaborate fantasy world, or he was jailed on other offences, or he is dead.

A police taskforce codenamed Spectrum identified 27,000 suspects and received 10,000 tipoffs. They checked 30,000 houses and arrested 73 people, but none was identified as Karmein's killer.

In 2010 the case was reviewed and all the evidence is intact. All that is needed now is a name. I recently came across an ex-cop known to have molested children who fitted the Mr Cruel profile. But he had been investigated and cleared. Another dead end.

If the first 48 hours are considered the most important in a homicide investigation, then the case of Jane-Thurgood-Dove may have been doomed from the start.

Jane, 34, was shot dead in front of her three children in her Niddrie driveway and the suspicion soon fell on a serving policeman obsessed with the married woman.

He fitted the description of the pot-bellied gunman and wanted to buy a cemetery plot next to her's. He built a memorial to her inside his house, his mobile phone number and computer password were derived from her date of birth, and to top it off he failed a lie detector test when asked, "Are you responsible for the death of Jane Thurgood-Dove?"

The case, while short of justifying laying charges, appeared compelling except for one flaw. He didn't do it.

For five years police pursued the wrong man just as the killer had pursued the wrong woman, for the paid hitman was supposed to kill another blonde mother who lived in Muriel Street. The target was Carmel Kyprianou, whose crooked husband, Peter, had a habit of making enemies.

One was former Crown solicitor Philip Peters, who had been caught planning to abduct and torture Kyprianou over a $200,000 fraud scam.

Jane was killed in November 1997, just months after Peters was released after serving time for the abduction plot.

Once the policeman suspect was cleared, the murder theory became that a drug dealer Peters met in prison was the go-between for the murder contract.

The dealer, who once worked for Australia's biggest amphetamine syndicate, used his connections to pull together a hit team from Geelong bikies. Police say the triggerman was former Rebels Motorcycle Club bikie Steven John Mordy, while his mate, James Ian Reynolds, organised the getaway car.

Before police could move Mordy died of natural causes and Reynolds in a boating accident, meaning the case appears to be at a dead end. But, and it is a big but, the go-between is still alive and there is a million-dollar reward on offer.

And in the underworld, greed is a bankable currency that can sometimes prick the conscience of even the most callous crim.

Crime Stoppers: 1800 333 000.