Thirty-five-year-old Margaret Tapp who was found dead in her bed at her home in Ferntree Gully. But the Tapp double murder was never famous. Which is extraordinary, because it was as savage and sinister a crime as the one committed at Easey Street - where two young women were stabbed in their Collingwood house in 1977, leaving a toddler crying in his cot. Or what happened when a man abducted, and later murdered, Karmein Chan from her parents' Templestowe home on April 13, 1991. In fact, two mysteries surround the Tapp murders. Not just the obvious one - who did it? - but why it was forgotten so quickly and ignored so long. The Tuesday night of August 7, 1984, was just another quiet one in the suburbs. Late that night an unknown man strangled Margaret Tapp, a 35-year-old nurse and part-time law student, and molested and killed her nine-year-old daughter Seana in their house at 13 Kelvin Drive, Ferntree Gully. The killer surely strangled Margaret first. It's inconceivable she would not have fought to save her little girl.

A neighbour heard a sound like a muffled scream just after 11pm. A little later, her dog growled. People across the road heard Seana's normally quiet spaniel bark and howl around midnight. Next afternoon, Tony Blackwell, a young man going out with Margaret's sister, called in. He picked up the newspaper in the driveway, knocked on the door and left, puzzled that Margaret's car was there but no one was home. Margaret was a striking and vivacious woman with many admirers. About 6pm that day, one of them, a Scottish-born carpenter called Jim, came around to go to the opera - a date she had arranged. When no one answered, he opened the back door, which had a faulty lock. He found Margaret's body in bed beside a book she had been studying. He hoped it was suicide and that Seana had been staying at a friend's. She wasn't. He rushed to the neighbours, shocked and sobbing, to call for help. Local police got there first, then two homicide detectives. It looked like ''a domestic'' and as usual, whoever reported finding the bodies was top of a shortlist of suspects. The carpenter was interrogated at length but his story was sound - and his innocence would (much later) be verified by DNA tests. The same went for Margaret's ex-husband, Don Tapp, who had a watertight alibi.

The investigation foundered as it became clear it wasn't just another simple domestic. The trouble was that by the time the usual suspects were cleared - notably Jim the carpenter - the trail had gone cold and split a dozen ways. Nothing has changed that in a quarter century. By the end of week one, the list of potential suspects was long and getting longer. Some have apparently been cleared by DNA testing in recent years. But when your correspondents first raked over the case on the 20th anniversary of the murders in late 2004, it seemed that only three suspects had been DNA tested up to that date. In 1984, of course, when DNA technology was still almost science fiction, fingerprints were the best investigative tool. Several prints were found in the house but eliminated because they belonged to friends or relatives who had legitimately been there. There were no ''stranger'' prints, meaning either the killer was lucky - or knew to avoid leaving prints at the scene. Police found two different types of hair on Seana's clothing and bedclothes: a long blond hair and shorter grey ones that might have been tinted. But they could have come from anywhere, any time, and did not identify a suspect.

That left two more clues that might still trap the killer: semen stains on Seana's nightdress, and Dunlop Volley tennis shoe prints in her mother's bedroom and the bathroom. The shoe size did not match that of Margaret, Seana, or anyone else police could place at the scene. They concluded the killer must have been wearing the Volleys. A lot can happen in 26 years. People change their looks, names, cars and addresses - but not their DNA or their feet. Whoever finds the perfect match will nail the killer. DNA is a blessing for most potential suspects because for every offender it pinpoints, it clears all other suspects for the same crime. It must have been a relief for a former senior policeman - a friend of the victims' family - when the homicide squad finally came calling with a swab kit. Until that happened, some people wondered why the policeman had been effectively ''written out'' of the case in the first few days, a fact that would stay hidden for 20 years. Margaret's sister Joan raised the retired policeman's name the day the bodies were found. Margaret had told her only a few days before that the man - who knew their father - had been visiting and bringing gifts.

Margaret reportedly had been annoyed at the older man's attentions. Not so annoyed, though, that it had stopped her visiting his remote country property, where he had taken a photograph of her and Seana and her teenage son only months before her murder. Others have been cleared, perhaps belatedly. Margaret Tapp's brother, Lindsay Nelson, was surprised the system took more than 20 years to rule him out. Nelson thinks police have since also tested a former workmate of his, a violent man with a record who had once delivered furniture to the Tapp house. But for every suspect cleared there are more who deserve to be. Topping the list is a family that lived in Kelvin Drive in the 1980s. The kind-hearted Margaret let one of them mow her lawn and clean her car - a decision that worried neighbours because of the youth's strange behaviour. For 20 years after the murders neither he, his three brothers nor his stepfather were eliminated by forensic testing. Nothing suggests that situation has changed. A sister of the same family lived in a caravan beside the house with a boyfriend later jailed for rape. Then there was the instructor who gave Margaret free truck-driving lessons, and the Shell employee who had gone out with her. And fellow law students and people with whom she worked. One of the investigators' problems was that Margaret Tapp was a liberated and highly social woman. Her friends say she had affairs with four or more doctors at a hospital where she had worked, as well as a long-term, stormy relationship with a married doctor who lived near Olinda and was later killed in a road crash. All deserve to be cleared - discreetly, if need be.

In a case with so many loose ends, no one lead is better than another. But the fact that police found a pizza in the Tapps' oven poses questions. Was it delivered? If so, did the delivery man wear Volley sandshoes? Postscript: In July 2008 prisoner Russell John Gesah was charged with the Tapp murders. The charges were withdrawn two weeks later when it was discovered the DNA sample had been contaminated.