Forensics: What Bugs, Burns, Prints, DNA and More Tell Us About Crime

by Val McDermid

The decision on whether the death in question is suspicious rests with a plain-clothes officer of the rank of Detective or above. Once the detective has determined that it could be a homicide, the scene is preserved for the CSIs.

‘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.’

Nowadays, gun barrels have spiralled grooves – ‘rifling’ – running down their inside, which encourages bullets to spin, and hence fly more accurately. Each gun model has slightly different rifling.

In 1835, Henry Goddard, a member of the Bow Street Runners (the first detective force in the UK), was called to the house of a Mrs Maxwell in Southampton. Joseph Randall, her butler, claimed that he’d been in a shoot-out with a burglar. He said he’d fought him off at the risk of his life. Goddard noted that the back door had been forced, and that the house was in disarray, but he was suspicious all the same. He took Randall’s gun, ammunition, moulds, and the bullet that had been fired at him, and discovered that they all matched: the bullet had a tiny round bump that corresponded to a similar-sized flaw in Randall’s mould. Confronted by this evidence, Randall confessed that he had staged the whole event in the hope of getting a reward from Mrs Maxwell for his bravery. This was the first time a bullet had been forensically traced back to a particular gun.

If they find fingerprints on a knife that’s been used in a stabbing, they don’t need to look for DNA on it. Peter explains, ‘We don’t want to be doing the really space age stuff when we can get the result we need by simpler and cheaper means.’

Peter is particularly fond of the UK National Footwear Database, which can link scenes together through footprints. He used it recently after finding a rare footprint at the scene of a sexual assault. The print had been found at a few other crime scenes throughout West Yorkshire, and the coincidence made the police focus their attention on the man who was eventually convicted.

Arsonists often leave matches behind, assuming they will burn away to nothing. But the powdered rock in a match head contains the fossilised remains of single-cell algae called ‘diatoms’. A diatom’s shell is made of silica, which is abrasive enough to help you strike the match, and tough enough to endure extremely high temperatures. Each of the 8,000 known species of diatom has a unique shell structure, identifiable through a microscope. Different brands make their matches using powdered rock from different quarries. If forensic scientists can spot the diatoms, they can identify the match brand. Then a search of a suspect’s pockets or CCTV footage from local shops can provide incriminating evidence.

They asked the parents of thirty children to set off the smoke alarms in their properties at random hours of the night. ‘Eighty per cent of these children did not wake up, even though some of them had the alarm in their bedroom.’ The variable frequency detectors designed to address the problem of heavily sleeping children seldom worked. Some of the most effective alarms are reported to be the ones that allowed the mother to record a message herself: ‘So she says, “Get up!” and children respond to the pitch and frequency of her voice.’

At a crematorium bodies are reduced to ashes by exposure to 815°C heat for around two hours.

Even if a body can be hermetically sealed off, clues to its location may still be obvious. American police in Indiana searching for a missing person some years ago noticed a cloud of frustrated flies hovering above a covered well. The missing person had been murdered, and the killer had thrown their remains down the well. He had sealed the well up enough to prevent insects getting in, but not the slight odour of decomposition getting out. The flies acted like a swaying gravestone, drawn to a smell beneath that was far beyond the capabilities of the human nose.

This startling observation came to Linnaeus because of the pioneering work of Francesco Redi. In 1668 the Italian had proved with a series of experiments that maggots came from fly eggs. Before Redi, the presence of maggots in corpses was presumed to be the result of spontaneous generation.

He wants to find the oldest specimens because they reveal when the flies first found the corpse, and so indicate the minimum time elapsed since death.

Maggots cannot tell us when a murder took place. They can indicate when flies laid eggs on a corpse, and that reveals the point by which the person was definitely dead. In the warm summer months it would be possible to narrow that window to, say, Friday, and possibly, as deductions become increasingly refined, to Friday afternoon. But to expect an entomologist to give a definitive time of death to the hour would be like asking a weather forecaster in November to promise a white Christmas. The range of variables thwarts that level of accuracy.

Then someone had the bright idea of sending the cases off for a toxicology examination. The results were startling. The cases contained traces of heroin metabolites. There’s no history of pigeons ingesting heroin, so further tests were ordered. Martin explains: ‘Maggots feed in a soup of DNA, and they have spines on their bodies which tissue gets lodged in. The pupal case is the old skin of the maggot and may still have human tissues on it.’ When the pupal cases were examined further they revealed traces of human DNA which matched that of a known drug user who had been reported missing. On the basis of this and other evidence, the owner of the house was convicted of murder and jailed for life. He had disposed of his victim, but could not silence the insect witnesses.

In a recent case in the US, the movements of a suspect were identified by the insects splattered on the windscreen of his vehicle.

‘To begin depriving death of its greatest advantage over us, let us adopt a way clean contrary to that common one; let us deprive death of its strangeness; let us frequent it, let us get used to it; let us have nothing more often in mind than death.’ Michel de Montaigne, Essais (1580)

The poet John Donne reminds us that ‘Any man’s death diminishes me, because I am involved in mankind’.

The very word ‘autopsy’ derives from the Ancient Greek for ‘seeing for oneself’. An autopsy is a medical attempt to satisfy that profound curiosity. The first known forensic autopsy took place in 44 BC, when Julius Caesar’s doctor reported that, of the emperor’s twenty-three stab wounds, only the one between his first and second ribs was fatal.

When Vesalius published his landmark book on anatomy, De Humani Corporis Fabrica (On the Fabric of the Human Body), in 1543 he dedicated it to the Holy Roman Emperor, Charles V, whose reign also saw another landmark in forensic medicine. For the first time in the history of the Holy Roman Empire, rules of criminal procedure were enacted. They regulated which crimes should be regarded as serious, allowed for the burning of witches and, for the first time, gave the courts the power to order investigations and inquisitions into serious crime. Known collectively as the Carolina Code, crucially for forensic medicine, they required judges to consult surgeons in cases of suspected murder.

Those authors included the French barber surgeon Ambroise Paré, sometimes called ‘the father of forensic pathology’. He wrote about the effects of violent death on internal organs, explained the indications of death by lightning, drowning, smothering, poison, apoplexy and infanticide, and showed how to distinguish between wounds made on a living and a dead body.

The symptoms of rigor are useful to the pathologist for about two days after death because of a recognised cycle. At first a body relaxes completely, then after three or four hours the small muscles of the eyelids, face and neck begin to stiffen. Rigor progresses downwards, from head to toe towards the larger muscles. After twelve hours the body is completely rigid and will stay fixed in the position of death for around twenty-four hours. Then the muscles gradually relax and stiffness goes away in the order in which it appeared, starting with the smaller muscles and progressing to the larger ones. After a further twelve hours or so all the muscles reach a state of complete relaxation.

During the autopsy he searched every inch of Mrs Barlow’s skin with a magnifying glass. Eventually he found two tiny holes consistent with injection needles, one on each of Mrs Barlow’s buttocks. The symptoms that Kenneth had said his wife was suffering from were those of hypoglycaemia (low blood sugar), which made David Price suspect that he had injected his wife with a lethal dose of insulin. There were no tests for insulin at the time, so Price took tissue from around the injection points on Mrs Barlow’s buttocks and injected it into mice. They quickly died of hypoglycaemia. Barlow was found guilty of murder and given a life sentence.

Over the next dozen years Mary Ann became the most prolific female serial killer in British history. Although it will never be known exactly how many people she poisoned with arsenic, she likely murdered her mother, three of her four husbands (the other one refused to take out a life insurance policy), a lover, eight of her twelve children, and seven stepchildren – at least twenty people in total.

In 1918 Charles Norris set up the first organised medical examiner system in the world when he became New York City’s first Chief Medical Examiner, responsible for investigating the bodies of people who had died unnaturally or suspiciously. Previously, forensic pathology had been the preserve of ‘elected coroners’, who were generally barbers or undertakers or worse. Forensic historian Jurgen Thornwald counted ‘eight undertakers, seven professional politicians, six real-estate agents, two barbers, one butcher, one milkman [and] two saloon proprietors’ serving as elected coroners in New York between 1898 and 1915. The system was incompetent and corrupt.

Something that became clear to Robert over time was that drug concentrations change significantly after death in most other parts of the body. ‘Interpreting the results is not at all straightforward,’ he admits. The scientific consensus used to be that ‘living blood’ gives the same toxicological results as post-mortem blood. ‘Nowadays we know that’s not true. You have to look at it with a great deal of caution. It’s very, very difficult.’

The thigh muscle is the most stable tissue in the body, making it a good place to find traces of poison.

Smith’s final report in 2005 estimated that Shipman had murdered 210 of his patients, with a possible further 45, making him the most prolific killer ever convicted.

If ricin is swallowed, its symptoms are nasty but not fatal. But if it is injected or inhaled or absorbed through the mucus membranes, a dose the size of a few grains of salt will kill an adult man. Ricin inhibits the protein synthesis of cells, causing cell death, and damage to the major organs. There is a delay of a few hours before the appearance of symptoms, which include high fever, seizures, severe diarrhoea, chest pains, breathing difficulties and oedema. Death ensues within three to five days; there is no antidote. Poisoners have favoured it over the years because, like arsenic, its symptoms mimic natural causes.

Our fingerprints are part of us from before birth; they first appear in the tenth week of pregnancy, when the foetus measures only 8 cm.

Even the prints of identical twins differ. In all the years that fingerprinting has been practised, no one has yet found two identical clear and complete prints belonging to two different fingers.

An American experiment in 2006 showed that even experienced fingerprint experts can be swayed by contextual information. Six experts were shown marks that each one had analysed before. But this time they were given certain details about the case – that the suspect was in police custody at the time the crime was committed, for example, or that the suspect had confessed to the crime. In 17 per cent of these secondary examinations, the experts changed their decision in the direction suggested by the information.

Traditional genetic fingerprinting had relied on bodily fluids and hair, but by 1999 the team that Gill was part of was using PCR to develop a much more sensitive method, known as ‘low copy number (LCN) DNA profiling’. To get an LCN profile they needed only a few cells from a potential suspect. Whether it was a speck of dead skin, the sweat from a fingerprint or the dried saliva from a postage stamp, the required amount of bodily substance had spiralled down from the size of a ten pence piece to one millionth of a grain of salt.

As with a real fingerprint, a genetic fingerprint should not be enough to secure a conviction on its own. According to Gill, ‘DNA doesn’t lie. It’s an exceptionally good lead and exceptionally strong evidence but there is human interaction in the process [of profiling]. So the error rate is exceptionally low but it’s not zero … DNA shouldn’t be a lazy way to not do an investigation.’

The first live case to be cracked using familial searching came in 2004. Michael Little was driving his truck under a motorway overpass when someone threw a brick from overhead. It crashed through the windscreen and struck Little in the chest. He managed to steer his truck on to the hard shoulder before succumbing to a fatal heart attack. When scientists fed the LCN DNA from the brick into the database, it produced no direct match, but a familial connection led them to Craig Harman, who admitted his crime and was sentenced to six years for manslaughter. For Detective Chief Inspector Graham Hill of Surrey Police, there was only one reason a conviction was secured: ‘There is no doubt in my mind that without this groundbreaking technique this crime would have remained undetected.’

Sue is director of the Centre for Anatomy and Human Identification at the University of Dundee. At the heart of her job in the field is the recovery and identification of skeletal remains. Are they human? What sex, what age, what height, what ethnicity? When did death occur? Why? If a corpse is intact and not too decomposed, a pathologist may be able to answer these questions. If not, a forensic anthropologist is needed to analyse not just the bones but all the ‘human remains’ left behind: hair, clothing, jewelry, any of the many items we collect and carry with us every day.

The first recorded instance of an anthropologist featuring in a criminal trial was in 1897 in America.

In general, it’s more uncommon for missing children to be recognised from photos than adults, despite the greater media coverage that they get, because their unformed faces are more similar to each other. Only one in six missing children is found because someone calls the authorities after seeing that child’s picture, according to the National Center for Missing and Exploited Children, who release thousands of images of missing children every week in the US.

Our brains are highly skilled in recognising minute differences in other people’s faces and we can identify hundreds of them as a result. At only five weeks old, babies can distinguish their mothers’ faces. And 2.5 per cent of people grow up to be ‘super recognisers’, capable of identifying nearly every face they have ever seen.

The shape of the face depends on the twenty-two bones of the skull. The complex shape of these bones, and to a lesser extent the muscles that are attached to them, explains the variation between individual faces.

All we can know about the ears is where they were and whether they had earlobes; although, in life, every pair of ears is as unique as a fingerprint.

In the Gilroy case there was no DNA. The scratches on his arm would not have been enough. He was convicted because of unusual mobile phone activity, CCTV video and images from road-side cameras.

Cloud computing presents other difficulties for the forensic expert. Software like Dropbox, which keeps files synced across devices, enables users to overwrite and change files on one device from another device anywhere else in the world. Angus calls this a ‘massive benefit to the end user but, from an investigative point of view, if somebody has made a change on their computer in their house on this side of the country, and their laptop in the other house on the other side of the country is still switched on, Dropbox changes the content on the laptop, meaning I cannot tell which house you were in.’

David Canter came up with a circle hypothesis: if you draw a circle with a circumference going through the sites of the two crimes furthest apart from each other, the culprit’s home will likely be near the centre of that circle. Research has shown this to be true of the majority of criminals who strike more than five times. Canter has found that a serial killer can usually be found living within a triangle formed by the sites of his first three murders, as Duffy was.

As quickly as possible a CSI uses tape to lift any suspect fibres or hairs off the jacket for analysis. Then she puts the jacket in a plastic evidence bag and sends it off to a laboratory scientist who looks for things like bloodstains. After running appropriate tests, the scientist rebags the jacket and stores it ready for a possible appearance as a courtroom exhibit. If the scientist can’t find anything useful, the jacket will go into a warehouse to await the next scientific breakthrough that might produce useful evidence, such as extra-sensitive DNA testing.

Prosecutors serve the state. Their links with the police give them a head start on the defence, but the prosecution must share all its findings with the other side before a trial. The principle of sharing evidence has to do with the legal concept of ‘due process’, whereby the prosecution may not withhold information that would help the defence. Without due process a fair trial is impossible.

The Foundation for the Study of Infant Deaths stated that in the UK second crib deaths in the same family actually occur ‘roughly once a year’.

For the sake of due process, the defence lawyer of an accused rapist must have access to all of the same medical records of the complainant as the prosecution lawyer, ‘Women are shocked when they find out,’ explains Fiona. ‘They think, “How did they get that?” The defence lawyer will say, “Is it the case that you were on tablets, let’s see, oh, tranquillisers, about three years ago because you had a bout of mental health issues?” And before you know it they are creating a story of this un-credible person who probably can’t remember very well and who perhaps is still taking tablets. For one reason or another, the most vulnerable witnesses are the ones with the longest medical records, and the defence have a field day. Complainants do have the right to refuse to pass over their records but they often don’t because they haven’t properly grasped the significance of revealing them.’

A 2014 study by legal experts and statisticians from Michigan and Pennsylvania found that 4.1 per cent of prisoners sentenced to death in America were innocent.

It was only in 2008 that Jed Rakoff, a federal judge in New York, finally held hearings to look into the status of ballistics evidence. He suggested that it had been more reliable in the days when bullets were made from individual moulds, but was much less so in the era of mass production. ‘Whatever else ballistics can be called,’ he said, ‘it cannot fairly be called “science”.’ In the wake of Rakoff’s study, some American jurisdictions will no longer allow ballistics experts to testify to a conclusive match, and defence attorneys have much to work with in challenging ballistics evidence.