Missing link?

The case of Stephen Lawrence raises many interesting and important issues covering the development and use of forensic science over the last 19 years. Key forensic evidence based on both fibre and bloodstains was presented at the Old Bailey in December last year in the case against co-defendants Gary Dobson and David Norris, who were charged with Stephen Lawrence's murder.

The guilty verdict was a culmination of years of investigation and a failed criminal and civil trial. A not-guilty verdict during the first criminal trial in 1993 was based on the Crown Prosecution Service arguing that there was 'insufficient evidence to provide a realistic prospect of a conviction'. The review of the evidence, following a cold case review of a pinhead-sized bloodstain on the collar of Gary Dobson's jacket with a DNA profile matching that of Stephen Lawrence, allowed the latest criminal trial to take place. The double jeopardy rule means that pre-2003 a person previously acquitted cannot be retried for the same crime. Post-2003, following the murder of Stephen Lawrence, the Macpherson report recommended the double jeopardy rule should be abrogated in murder cases and this means that if compelling new evidence is found then the previous acquittal can be quashed and the person re-tried. This came into effect within the Criminal Justice Act 2003 and came into force in April 2005. Any retrial must be approved by both the Director of Public Prosecutions and the Court of Appeal.

Further, fibres matching the clothing of Stephen Lawrence were also found on tape lifts taken during the original examination of their clothing. In forensics we call this a two-way transfer: there was material to identify a link between the defendants and Stephen Lawrence and vice versa. This finding therefore established the link between the people involved, and, while 'source' attribution (i.e. where the fibres and blood could have come from) does not necessarily identify the 'activity' level (i.e. how were the fibres and blood transferred), the prosecution case was that they were deposited at the time the offence took place.

It was reported, and accepted by the Crown testing laboratory, that the bags containing the various evidence were stored together and some bags had been left open during storage. The defence argued that transfer of small blood flakes or fibres to the clothing of Mr Dobson and Mr Norris could have occurred either during storage with exhibits taken from Mr Lawrence, or during the initial and subsequent examinations undertaken by the Crown to account for innocent transfer of these materials.

One such method of transfer during the examination of the items was during testing for saliva, a procedure known as the Phadebas test, which involves placing a large sheet of porous paper onto the item to be tested and dampening it to allow any body fluids to soak into it. It was argued that a small blood flake present on the surface of the jacket, previously innocently transferred during storage with items from Mr Lawrence, could have been dampened and then soaked into the weave of the fabric. In this way it could be argued that the small spot of blood was deposited at the time of the offence when the blood was wet. Given the small size of the bloodstain and the nature of the fabric it would be important to consider whether a specific blood pattern was present to identify how the blood was transferred, wet at the time or re-wetted at a later date.

The advances over the last 19 years in forensic science have been both procedural, in the way forensic evidence is collected, stored and dealt with at both the crime scene and laboratory, as well as technical, in the ability to test for ever-smaller amounts of DNA from ever more degraded samples. In 1996, the latest version of the UK's DNA testing system, designed by the government's forensic laboratories, SGMPlus, which is now the current kit used by the UK Criminal Justice System and National DNA Database, was introduced, which generates statistical probabilities where a full DNA match is found of one in a billion (a thousand million). The advent of low template DNA profiling (LCN) in 1997, but only routinely used in mainstream cases since 2001, where a DNA profile can theoretically be obtained from a small number of cells, means the transfer of DNA via coughing, sneezing or via secondary means, via an intermediary surface or person known as secondary transfer, is a very real possibility and must be addressed if it is a realistic possibility and the opportunity has potentially arisen.

Similar challenges

However, the challenges faced by today's investigators and forensic laboratories are not fundamentally different to those faced by experts in 1993 at the time of the original trial. Consideration of potential contamination of exhibits and locating body fluids are the fundamental tenets of forensic science. It does not matter if the ability to test a small sample of blood is possible if we do not first locate it, and second if we cannot be sure that the source of DNA was in fact not the result of innocent contamination. This is especially important where the sample being tested is a bloodstain the size of only 0.5mm by 0.25mm as in the Lawrence case.

Similarly, the issue of fibre evidence is reliant upon the fact that there has been no cross-contamination between exhibits. For this reason items are routinely examined in separate areas of the same building, or in different buildings, utilising stringent airflow control, to ensure this is not an issue to the courts. A jury must convict someone based on reasonable doubt and any aspect that suggests that forensic evidence cannot be relied upon must put doubt as to its reliance to the jury in their deliberations. In reality the judge will direct the jury that they must be sure a defendant is guilty to deliver a guilty verdict.

As with DNA evidence, today the ability to test fibres for potential inclusion or exclusion via both spectroscopic and chemical means is more advanced than 19 years ago. Again, however, if the answer to the question of whether it is possible that fibres may have been inadvertently transferred between exhibits is 'yes', then the reliance on the findings can only be much reduced at court, if at all. To allay fears of innocent transfer, the number of fibres recovered, the type of fibres and the ease of their transference and persistence will be key to assessing how likely it is that an inadvertent transfer could have taken place.

Further, the mix of two and three different fibre types from Mr Lawrence's clothing found on Mr Norris and Mr Dobson's clothing respectively means that the chance of a spurious match would be, in normal circumstances, less likely in combination than on their own. Also, the issue of how common a specific item of clothing, and its component fibres, is in the environment will alter the weight of evidence placed on finding matching fibres; white cotton fibres are ubiquitous and as such finding matching fibres of this type would not be considered important. If fibres are found, the impact on the court of whether exhibits had potentially physically touched, and fibres innocently transferred during storage, is unknown.