Forensics
Val McDermid
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In 1794, Edward Culshaw was murdered with a pistol shot to the head. Back then, pistols were muzzle-loaded and a wad of crushed paper was tamped down to secure the balls and the powder in the gun. When the surgeon examined the body, he found the wad inside the head wound. He opened it up and it was revealed to be the torn corner of a ballad sheet. The murder suspect, John Toms, was searched. In his pocket was a ballad sheet, the torn corner of which matched exactly the pistol wad. At his trial in Lancaster, Toms was convicted of murder.
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We crime writers sometimes like to claim our genre has its roots in the deepest recesses of literary history. We claim antecedents in the Bible: fraud in the Garden of Eden; fratricide by Cain of Abel; the manslaughter of Uriah by King David. We try to convince ourselves that Shakespeare was one of us. But the truth is that crime fiction proper only began with an evidence-based legal system. And that is what those pioneering scientists and detectives bequeathed us.
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In 1929, Locard published a paper on the identification of tobacco by studying ashes found at a crime scene, 'The Analysis of Dust Traces'. He wrote a landmark 7-volume textbook on what he called 'criminalistics', but probably his most influential contribution to forensic science is his simple phrase, known as the Locard Exchange Principle: 'Every contact leaves a trace.' He wrote: 'It is impossible for a criminal to act, especially considering the intensity of a crime, without leaving traces of his presence.'
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It might be fingerprints, footprints, identifiable fibres from his clothing or his environment, hair, skin, a weapon or items accidentally dropped or left behind. And the converse is also true – the crime leaves traces with the criminal. Dirt, fibres from the victim or the scene itself, DNA, blood or other stains.
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'People think of a crime scene, singular. But often we can end up with five or six relevant scenes for a homicide – where a person has been killed, where the suspects have gone afterwards, a vehicle that the suspects have travelled in, where the suspects are arrested and, if the body is removed, where it ends up. All of those different scenes need to be processed separately.'
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Because different insects don't all turn up at the buffet together; there is a predictable order to their arrival. When the entomologists are called in they use their knowledge of this succession to estimate time of death. And so the insect kingdom helps the dead victims to provide unwitting but convincing evidence against their killers.