Forensics

by Val McDermid

  British CSIs use various methods for recovering prints in a logical sequence, starting with the least destructive. The order of actions is set out in the Home Office’s Manual of Fingerprint Development. First the CSI examines surfaces for patent prints, like the bloody one on the doorframe at the Rojas’ house; if necessary, she photographs them. Then she shines lasers and ultraviolet light on to surfaces to illuminate latent marks, and make them photographable. If special lighting doesn’t work, she delicately brushes a dark powder over the mark, takes a photo, and then presses an adhesive strip over it. Once the strip has been peeled off and pressed on to a white card, it is known as a ‘last’. This is Henry Faulds’ classic way of recovering fingerprints from a crime scene, and it is still the most commonly used method today. If a print remains stubbornly invisible, as it might well do on more porous surfaces, the CSI can use a range of chemicals which react with the salt and amino acids in human sweat, to make it visible.

  The photographs and lasts are then sent to a fingerprint officer, who decides whether they contain enough ridge detail to make identification possible. If the print isn’t too smudged or incomplete, the officer compares it to fingerprints of neutrals – that is, people who had every right to be there and who are not suspects – including victims and police officers, before looking at prints from potential suspects. This is inevitably a subjective process. If the officer judges that none matches, she scans the print and encodes it into a geometric pattern. Then she runs an automatic search using a national database, such as the UK’s IDENT1, which holds the prints of around 8 million people.

  IDENT1 is the modern equivalent of Edward Henry’s pigeonholes. Both IDENT1 and the FBI’s database use a slightly modified version of Henry’s classification and identification system. A computer program asks the fingerprint a series of questions, such as, ‘How many whorls have you got?’ Each answer is given a numerical value – ‘two whorls’ is two points. The values are strung together to give the print an overall code. IDENT1 then compares this code to the 8 million others it contains, and presents the officer with the closest ten or so prints.

  She must now judge if any of them are a positive match. This, again, is a subjective process. Once she has found a similarity in the overall pattern of the ridges, she concentrates on tiny distinguishing points known as ‘minutiae’, which include points where ridges start, end and join together; where they sit independently; and where they form little bridges between two other ridges.

  In 1901, when Scotland Yard established its Fingerprint Bureau, officers like Charles Collins needed to find at lease twelve minutiae in agreement before they could testify to a match in an English court. In 1924 this was increased to sixteen points, which was higher than in most other countries. At the time most fingerprint experts thought that eight was enough. If an officer found between eight and fifteen points, they usually reported it to the police, because it might give them a valuable lead. But by 1953 all the UK police forces had adopted the 16-point standard.

  Since the Stratton brothers case, belief in fingerprinting has gone from strength to strength amongst the civilians, judiciary and police forces of the world. And for many, including large numbers of experts, it has an aura of absolute infallibility. As Jim Fraser writes in Forensic Science (2010): ‘In the view of most marks examiners, identification of an individual by fingerprints can be done unequivocally, that is, with 100% certainty.’

  When a print is clear, the chances of an officer making a wrong call are next to nothing. But when a print is smeared, overlaid with other marks, made in blood, one officer may see points of agreement that another does not. A case in 1997 tested the subjective nature of fingerprinting to breaking point. On 6 January, the body of Marion Ross was discovered in her house in Kilmarnock, Scotland. She had been the victim of a horrific attack: her injuries included multiple stab wounds and crushed ribs, and a pair of scissors was found embedded in her throat. CSIs set to work recovering evidence, and found more than 200 latent finger marks in Marion’s house, which they sent to the Scottish Criminal Records Office to be checked off against the neutrals – paramedics, doctors, police officers.

  The finger mark that became the eye of a perfect storm was a left thumbprint on the bathroom doorframe. Despite it being quite badly smudged, a fingerprint officer confidently identified it as belonging to 35-year-old Detective Constable Shirley McKie, who was supposed to have remained outside the house to preserve the scene while investigators gathered evidence inside. She would have had to leave her post in order to touch the door; that would have been gross misconduct.

  Officers are thoroughly trained how to treat crime scenes; CSIs always wear protective gloves so they don’t damage the fragile traces left by criminals. Because of the seriousness of the case, a further three experts at the Scottish Criminal Records Office examined the thumbprint, and confirmed it to be McKie’s. It looked as if the detective had indeed abandoned her post.

  Meanwhile, the prime suspect for the murder had been identified as David Asbury, a 20-year-old handyman. Investigators had found his finger marks in Marion’s house, and her prints on a tin box in his. Asbury explained that he had recently done a job in Marion’s house, hence the marks. But detectives thought there was enough evidence to arrest him.

  At Asbury’s trial, McKie testified that she had not been inside Marion Ross’s house at any time, so the thumbprint could not have been hers. All the other fifty-four officers who had worked the crime scene corroborated her assertion. Nevertheless she was suspended from Strathclyde Police, and eventually dismissed.

  But that wasn’t the end of her nightmare. Early one morning in 1998 Shirley McKie was arrested. She had to get dressed under the watchful gaze of a policewoman. The officers took her to the police station where her own father, Iain McKie, had been commanding officer. She was strip-searched and shut in a cell. She learned she was to be charged with perjury, which carries the threat of an 8-year jail sentence. Her father’s long and distinguished police career had left him convinced of the integrity of fingerprint evidence. It was easier to believe that his own beloved daughter was lying than to doubt the experts. ‘People have been hung on a fingerprint,’ he reminded her.

  In May 1999 Shirley McKie was brought to trial at the High Court of the Justiciary. Two American experts who had examined the thumbprint maintained that the print was not hers. One said the ‘obvious’ differences took ‘only seconds’ to see. On this evidence the jury found McKie not guilty of perjury. In August 2002, David Asbury’s conviction for murder was also quashed by the Court of Criminal Appeal in Edinburgh – because of faulty fingerprint evidence. He had spent three and a half years in jail.

  After Shirley McKie’s innocence had been established, the Scottish Criminal Records Office and four police officers of the Strathclyde Police were accused of misconduct. McKie subsequently launched a damages case and in 2006 received a £750,000 settlement.

  But by then she had lost the job that she loved, spent years working in a gift shop and suffered from severe depression. Iain McKie now travels the world campaigning for better expert evidence to be produced in courts of law, and warning people about the entrenched attitudes of fingerprint experts.

  In 2001, the 16-point standard was scrapped in England and Wales, partly because of the McKie–Asbury fiasco, and partly because it wasn’t really a standard. If fingerprint officers had found fourteen points they would sometimes search for two more, to get a ‘match’. They were looking for similarities rather than differences, and that was dangerous. Since the sixteen points were scrapped no numerical standard has existed. But other experts very seldom challenge the individual decisions of fingerprint officers.

  Catherine Tweedy is one of the very few people alive today whose job it is to challenge fingerprint officers. On first impressions, she seems like the kind of teacher kids love because she brings out the best in them – interested, encouraging, knowledgeable. But five minutes in her company reveals something else: a steely i
ntelligence devoted to ferociously logical argument and a passion for getting things right. She has completed a range of fingerprint courses in the UK and abroad, including ‘Advanced Latent Fingerprinting’ with the Miami Police in Florida. She currently works as a fingerprint expert for a forensic consultancy based in Durham, mostly working as a defence expert, where it is her job to double-check a proportion of the fingerprint identifications made in the UK – albeit a smaller proportion than she’d like. ‘I’ve been doing this from the mid-1990s,’ she says, ‘and I’ve come to it as a scientist. I tear my hair out at the assumption people have made right the way through that it is an absolute rock solid science. It’s not a science at all. It’s a comparison.’ The rhetoric used to support forensic fingerprinting has always been scientific in tone. But Catherine Tweedy has spent twenty years trying to remind people that moving forward along a road to certainty is not the same thing as arriving at it; and that going in reverse is also possible.

  In 2006, the year of McKie’s settlement, Scotland followed England and Wales in scrapping the 16-point system. In 2011 the results of a public enquiry into the McKie–Asbury fiasco were published. It put the misidentifications down to ‘human error’ and not to misconduct on the part of the Strathclyde Police. It recommended that fingerprint evidence should from now on be regarded as ‘opinion evidence’ not fact, and thus should be treated by courts ‘on its merits’.

  But this message has not trickled down to all fingerprint officers, says Catherine Tweedy: ‘They are not being trained to think that an opinion is an opinion. Once you are trained to see things as facts it is extremely difficult to be pulled back to understand that there are shades of grey. You can’t be 100 per cent certain in a lot of cases because you only get a fraction of a fingerprint.’

  Even when the fingerprint is correctly matched with an individual, mistakes can sometimes be made when the investigator tries to work out what this means. In one of her first cases, Catherine dealt with Jamie, a 14-year-old boy charged with burgling a house in Northern Ireland. His handprint had been recovered on a windowsill in the bathroom. When she met him, he said he’d never entered the house in his life. Catherine went inside and could see why that might be true. The house was such a stinking mess that it was tough to carry out any kind of exhaustive examination. When she looked at the handprint, she saw that it was a clear match for Jamie’s. But if someone had climbed in or out of the bathroom window, they would have left footprints in the bath or sink as well as disturbing all the junk underneath the windowsill. And there was no evidence of that.

  The SOCO hadn’t been into the other rooms or looked at the two external doors. Catherine carried out her own examination and she couldn’t find any other evidence to link him with the inside of the house.

  Fired up by Catherine’s work, Jamie’s defence team found out that the owners of the burgled house had heartlessly kicked their daughter out of the house and on to the streets on her sixteenth birthday. She had gone to stay with friends for a couple of weeks. Then, when she knew her parents were out shopping, she had returned home with her key, walked through the front door and collected her ghetto blaster, moneybox, some of her clothes and a few of her videos.

  When her parents returned home they spotted the missing items and called the police to report a burglary. The investigation had begun and ended with the palm print in the bathroom. No further questions had been asked. When Catherine Tweedy had Jamie’s friends questioned they told her that they used to play a game called Pirates round the back of the house. Pirates is a variation on the game of tag where players avoid being tagged ‘it’ by getting both feet off the ground. Jamie, it transpired, was a good climber. His best trick was to shimmy up the drainpipe and hang one-handed off the bathroom windowsill. Without Catherine’s tenacity, his agility might have landed him in jail.

  Some fingerprints are lifted in far more horrific contexts. On 11 March 2004, during the peak of the rush hour, ten bombs exploded simultaneously on four commuter trains in Madrid, killing 191 people and wounding 1800. The FBI suspected Al-Qaeda involvement.

  The Spanish police discovered an abandoned set of detonator caps inside a plastic bag that bore a single, incomplete finger mark. This was run through the FBI database, which showed up twenty possible matches.

  Spanish forensic experts search for clues in a blasted train carriage outside Atocha train station. The 2004 terrorist attacks were among the worst in Spain’s history

  One of those possibles was Brandon Mayfield, an American-born lawyer living and practising in Oregon. He was on the FBI fingerprint database because he had served in the US army. But, more significantly in counter-terrorist terms, he had married an Egyptian and converted to Islam. He had defended one of the Portland Seven, a group of men who had tried to go to Afghanistan to fight for the Taliban, albeit in a child custody case. But he also worshipped at the same mosque as them.

  The FBI decided that Brandon Mayfield was implicated in the bombing, even though his fingerprints were not an exact match, his passport had expired and they could find no evidence that he had travelled abroad for years. They began watching him and his family.

  Despite the Spanish police insisting that the fingerprint evidence should be rejected, the FBI agents tapped Mayfield’s phone, broke into his house and office, went though his desk and financial records, examined his computers and tailed him. When Mayfield realised he was under surveillance he panicked, so the FBI detained him to prevent him making a run for it. Two agonisingly long weeks passed before the Spanish police matched the fingerprints to the real culprit, an Algerian man named Ouhane Daoud.

  Mayfield sued the US government for wrongful detention, and in 2006 received a formal apology and a $2 million settlement.

  The FBI later acknowledged one of the problems in the handling of the Mayfield case was that their fingerprint experts had failed to separate the analysis and comparison stages of their examination. First of all the expert should analyse the mark in detail, describing as many minutiae as she can. Only afterwards should she examine possible matches and carry out a comparison. When analysis and comparison happen simultaneously, experts run the risk of finding matching minutiae because they are looking for them. In the view of Itiel Dror, a cognitive psychologist at University College London, ‘The vast majority of fingerprints are not a problem, but even if only 1 per cent are, that’s thousands of potential errors each year.’

  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. In other words, they couldn’t divorce themselves from the context and judge objectively. This kind of bias is less likely to occur in the UK, where forensic divisions are separated from others divisions in most police forces.

  In spite of the question marks raised by experts such as Catherine Tweedy, courts around the world continue to treat fingerprint as infallible and solitary finger marks still send people to jail. In her popular book The Forensic Casebook (2004), N. E. Genge states that ‘Examiners don’t think in any percentages except 100 and 0.’ But Christophe Champod, a Swiss expert in forensic identification, calls for fingerprint evidence to be treated in terms of probabilities – bringing it in line with other forensic disciplines – and that examiners should be free to talk about probable or possible matches. He has also called for the overall importance of fingerprinting to be downgraded: ‘Fingerprint evidence should be expressed by fingerprint examiners only as corroborative evidence.’

  If forensic science were a family, fingerprinting would be the greedy grandfather, hanging on to the best armchair, trying to exercise the sole right to pass judgement, unaware
that the times they are a-changin’. Only when the rest of the family understands that he sometimes gets people and places and anecdotes mixed up can his wisdom be treated with appropriate circumspection; then his contribution to the family can be regarded as a healthy and balanced one.

  SEVEN

  BLOOD SPATTER AND DNA

  ‘Will all great Neptune’s ocean wash this blood

  Clean from my hand? No, this my hand will rather

  The multitudinous seas incarnadine,

  Making the green one red.’

  Macbeth, II, ii

  Blood. It’s the key to life. Without it, we die. It’s the thread that runs through history, transferring property and power from one generation to the next. From the earliest times, man has understood blood both as a tribal marker and as an individual blazon. In some societies, inheritance flowed not from father to son, but from father to sister’s son, because you could be sure that your sister’s son was of the same blood as you. You knew for a fact his grandmother was your mother; you couldn’t be certain your own sons shared your blood.

  It’s also been at the beating heart of crime fiction from the beginning. When Doctor Watson first lays eyes on Sherlock Holmes, he is bent over a table perfecting a test for haemoglobin. Watson’s slowness in grasping the test’s brilliance makes the consulting detective fume. ‘Why, man, it is the most practical medico-legal discovery for years. Don’t you see that it gives us an infallible test for bloodstains. Come over here now!’ Then he stabs a needle into his own finger and uses the resulting drop of blood to show the test in action.