Suppose you killed a boy. Suppose you stopped the 12-year-old as he walked along a country path on the South Downs and stabbed him repeatedly with a steak knife in the back, chest and stomach. Suppose you threw the knife into the bushes and ran for a public toilet on the edge of the next village, where you could try to scrub off the blood.
Suppose you never got caught, even when the police went knocking at 75,000 homes. Then the years turned into decades. The police lost heart. They even lost the evidence for a while. The mother and father of the boy died, but you grew past middle age, still keeping your secret. Would you be able to live with that? Would you dare to suppose, at last, that you had got away with it?
You would be wrong. The killing of Keith Lyon happened 39 years ago, in May 1967. They called it the Summer of Love in happier places than Ovingdean in Sussex, where a village was mourning the shocking death of a bright boy. Memories fade; detectives retire. But now their successors have reopened the file and believe they have a hope of catching the murderer at last. This long-cold case is one of thousands now being revived because of extraordinary advances in forensic science.
DNA profiling is not new, but the number of crimes solved through it has quadrupled in the past five years. The latest breakthrough is that profiles can be made without body fluids such as blood or sperm. Scientists can take a piece of evidence - such as a 40-year-old steak knife - and lift from it a single cell of dry skin that may carry in its nucleus the genetic fingerprint of a murderer. "It is dust, basically," says Jonathan Smith, specialist adviser with the Forensic Science Service. "We can work with cells that have been ignored for years, not least because they are invisible to the naked eye. As long as they have not been got at by the damp or mould, or frozen, they could still be useful even after all this time."
The chances of a DNA match between two unrelated people is one in a billion. That does not prove guilt, but it does mean there are serious questions to be answered. Sussex Police announced last Wednesday that they had arrested two men in their 50s and taken samples for DNA tests. The results are not expected for weeks. The men have been bailed until November. "If you know who killed Keith," his younger brother Peter said last week, "please share your secret with the police."
Peter was seven years old when it happened. He stayed at home with his mother, Valda, on that Saturday afternoon, while Keith set off up the bridleway between Ovingdean and Woodingdean to visit a shop there. He had two shillings in his pocket and wanted to buy a geometry set. This was before men went to the moon, when clever children drew shapes with a compass or protractor. They played with marbles, not Gameboys. Long ago. The path Keith walked still crosses fields, though. Brighton has reached out to these villages on the cliffs, but the marina and the edge of the city are still hidden just behind a hill called Mount Pleasant. Heads of corn sway on one side of the bridleway; on the other the crop has been harvested or flattened. The local name for these fields, despite what happened, is Happy Valley. A rusty farm trailer sits by a gate that has been obsolete since the hedges fell apart. Police boots kicked back the undergrowth to look for clues here.
Keith's father, a professional musician, was preparing to play at the Metropole Hotel in Brighton when they gave him the terrible news that afternoon. "The murder ruined our lives," said Mrs Lyon. She became a widow in 1991, and died in October last year, still longing to know who had done it, let alone why.
Fragments of a burst red and yellow football lie on the chalky path. Children still play here. Witnesses speculated that a group of youths seen on the path picked on Keith because he was wearing part of his new uniform for Brighton and Hove Grammar. The police interviewed 2,000 children at 17 schools, but got nowhere. They now want to trace a family rumoured to have emigrated to Canada "hastily" after the killing.
Detectives did return to the case several times over the years, including appeals on the Crimewatch programme. But they also made a confession: sometime in the Seventies they had lost the knife. It was bagged and boxed as evidence - along with a cigarette butt, clothes and a blood-stained tissue - then mislaid. It was not found until 2002, when a sprinkler system was being fitted at John Street police station in Brighton. There in a locked and sealed basement room was the box.
"This is brilliant news," said Peter Lyon at the time, with hopes that new technology could get a DNA profile. It was not good enough then but is now, thanks to a technique called Low Copy Number profiling, which allows microscopic samples to be used. The murder weapon is believed to have been sent to the Forensic Science Service which operates 13 laboratories across the country. The nearest to Sussex is on the Lambeth Road in south London, in an anonymous office block. Security is tight. The lift to the laboratory is crowded with police officers in body armour escorting bags of evidence.
The scientists work behind a closed door that bears a sign saying "Alert! DNA Clean Room". They wear white coats, surgical-style caps and masks, dust cuffs and rubber gloves to move around the lab in which lights are bright and the white surfaces spotless. One scientist at a workbench examines a soiled white boot with a stiletto heel, pausing to draw on an electronic notepad. "Sketching the blood stains," explains Jonathan Smith.
A sample of the blood will be taken using a swab like a cotton bud soaked in distilled water. This will then be spun to extract the liquid, and chemicals added to isolate the deoxyribonucleic acid known as DNA that is found in almost every cell in the body. It carries the genetic code that determines characteristics such as the colour of our eyes or hair. This is translated by scientists into letters - A, C, G and T - and each person's code is three billion letters long.
Everyone has a unique code (except identical twins), but the difference is small: one letter in every 500. The technology is not yet good enough to allow scientists to compare every aspect of a suspect's DNA, so they concentrate on areas in the string of letters where there are expected to be variations. British DNA profilers compare 10 sections, plus the part of the code that determines gender. That makes it sound simple, but the bioscience is complex and relies on the judgement of those doing it, says Mr Smith, as we look at a computer screen full of mystifying wave patterns. "This is not a mechanical process but a patient, handcrafted one."
This, and the huge volume of work being sent in by the police, means it can take weeks to test for a match - although forces can choose to pay for a "premium service" that takes just 48 hours. "Undetected [new] cases do take priority because there is a chance the offenders may be caught quickly," says Mr Smith.
Profiles can be matched against the National DNA Database, which carries samples from crime scenes, volunteers and staff and almost every "active offender" in the country, according to the Home Office. There are more than three million profiles. But they include a disproportionate number of young black men, some children and 139,463 people who have never been charged or cautioned with anything. Critics say keeping such records is an invasion of privacy and a violation of human rights.
Profiles on the database have so far been linked to the scenes of 3,000 crimes including 88 murders, 45 attempted murders and 116 rapes. One of the cold cases it has revived is that of David Kirby, who raped a woman in Tunbridge Wells, Kent, in February 1989. He approached a couple in a car park, pointed a shotgun at the man's head and ordered them both to lie face down. Kirby bound their hands then raped the 18-year-old woman. Fifteen years later, a DNA profile was taken from the victim's jeans, and it matched that of Kirby - who had only recently been placed on the database after being arrested on suspicion of possessing indecent images of children. He pleaded guilty to the rape, as many do when confronted with a DNA match, and was jailed for 13 years.
In his gloomy office at the Lambeth block, Jonathan Smith opens a box of yellowed papers from a murder in 1970. The police often ask him to go over old paperwork to see if anything can be done. That is much more likely when material has been removed quickly and stored by forensic scientists, he says. "I could show you pictures of crime scenes where the police are standing around in wet raincoats puffing away at pipes. Horrifying." Countless hands may have grasped a murder weapon after fingerprints have been taken."How were they to know any different? But it does mean that if we do come up with a profile it might be a police officer or someone in the path lab."
So the Lyon case may be far from closed. The police have taken samples from a 56-year-old man from Brighton and a 55-year-old who lives in Manchester, but neither may provide a match. Keith Lyon would have been 51 by now. Each step on the dry path where he died kicks up a little chalky cloud. Far away, in a sterile laboratory, the only hope of catching his killer lies hidden in a tiny speck of dust.
How DNA can catch a killer
1. Even after many years, a murder weapon may still carry clues. The Forensic Science Service tests for deoxyribonucleic acid or DNA, a chemical with a unique pattern in each individual. Saliva, semen or rooted strands of hair can provide useful samples, as can white blood cells. The latest technology also allows DNA to be extracted from old skin cells that are invisible to the naked eye
2. There are 100 trillion cells in a human body. Inside each nucleus is at least one copy of the genome, the genetic code that determines who we are. The genome is divided into 23 pairs of chromosomes - long, tightly coiled strands of DNA
3. Each strand of DNA is a double helix with four building blocks called adenine, thymine, cytosine and guanine. These are called the DNA bases, and there are three billion of them in every genome
4. Scientists label the DNA bases as A, C, G or T. Each set of three letters corresponds to one of 20 amino acids, which make the proteins that build us. The combinations of letters are different for every person, laying down characteristics such as hair and eye colour
5. DNA differs between humans by no more than one letter in 500. Forensic experts examine 11 sections in the DNA chain whose lengths are most likely to differ.
They are compared with the same sections in the profile of a suspect, or matched against three million samples on the National DNA Database
6. The chance of a match between two unrelated people is one in a billion. Prosecutions in which DNA has played a part include Ian Huntley, who murdered the schoolgirls Holly Wells and Jessica Chapman in 2002. However, campaign group Genewatch says that even if the whole population was sampled, DNA would still only help solve 0.5 per cent of crimesReuse content