Solving the mystery

You’ve seen it on CSI but just how is the case solved? Our new series on forensic analysis will let you in on a few of the secrets used by forensic scientists to help solve a case. This month we look at gunshot residues

Gun crime can wreck lives and destroy communities. Fortunately, gun crime is a rare occurrence in the United Kingdom, but its impact on some inner city areas has been huge. Shootings are some of the most difficult cases for police to solve; eye witnesses are often reluctant to come forward for fear of reprisals or because of their own criminal activities. Forensic science has to be at the forefront of the police investigation and gunshot residue (GSR) examination is a key part of this.

GSR particles are invisible to the naked eye, which means specialist equipment and expertise are needed to identify them. GSR is produced by virtually all modern ammunition. The forensic scientist is most concerned with recovering and indentifying material created by ignition of the friction-sensitive primer.

When the trigger of a gun is pulled, this leads to the firing pin of the gun striking the base of the cartridge. The impact causes detonation of the primer material inside the base, which in turn ignites the propellant. As the propellant burns it generates very high pressure inside the cartridge, which forces the bullet out and onwards down the barrel of the gun.

Before firing, the primer is a mixture of several different chemicals, including a friction sensitive compound (usually lead styphnate), an oxidising agent (usually barium nitrate) and a fuel (usually antimony sulphide). After firing, these separate chemicals form particles containing lead, barium and antimony. These are highly characteristic of having originated from a round of ammunition.  Material from the bullet, the gun, the bullet jacket, cartridge case and the propellant are also incorporated into the cloud of residue that is produced. These tend not to be so useful to the forensic scientist, as there are other sources for these materials in the environment.

The exact composition of GSR can vary. For example, some Eastern European ammunition manufacturers will seal the primer into the cartridge using tin foil. When the round is fired, the tin can become incorporated into the GSR particles, thus giving a distinctive chemical composition of lead, barium, antimony and tin. In all, there are five basic GSR compositions which are frequently seen in forensic casework and several other types that are seen rarely. It is possible to examine a cartridge case from a crime scene, to see if it could have been the source of GSR found on a suspect’s clothing. However, many different cartridges will produce similar GSR, so this work only demonstrates potential associations, not definite matches.

If someone is arrested following a shooting incident, the police will often wish to recover GSR from their hands, face and hair. LGC Forensics and WA Products have co-designed a kit especially for this purpose, which contains a series of adhesive stubs, including some to use as controls to demonstrate the background environment is free from contamination. There are also comprehensive instructions, gloves for the police officer to wear, a worksheet and an evidence bag for securely sealing the samples. The kit is designed to be simple to use by police officers in busy custody suites. Another kit has been designed for recovering GSR from vehicles and scenes of crime, saving police the expense of calling out a specialist to do the work.

The police will also usually seize the suspect’s clothing for forensic examination. The clothes are carefully packaged for transportation to the forensic laboratory. At LGC Forensics, scientists from several different disciplines of forensic science will discuss the best way to maximise the evidence that can be gathered. For example, the surface of a glove may be sampled using adhesive stubs to recover GSR, then with adhesive tape to recover fibres that can be compared to those found on a recovered gun. The inside of that glove may be taped to recover the wearer’s DNA and any debris such as paint or glass flakes is collected for comparison with the crime scene. All these different types of evidence can be collected during the same examination, improving efficiency and reducing cost.

The strength of the evidence can also be multiplied when more than one type of evidence is found. For example, if a glove is found near the scene of a shooting, it may be examined to identify the wearer through DNA analysis. If a DNA profile is obtained, this may give the police vital intelligence about the identity of the gunman, which can be used to arrest and charge a suspect. However, when the case gets to Court, the defendant may accept that his DNA was present on the glove, but claim that he had dropped it near the scene a few days before the shooting took place. If the glove is also shown to have GSR on it, this gives a much stronger indication that the glove was actually being worn by the gunman.

Precautions against contamination are essential in the forensic laboratory. In the case of GSR, accidental contamination is a particular risk as the particles are invisible to the naked eye. The risk of contamination is minimised by preventing firearms users from entering the building and never carrying out examination of guns and ammunition on the same site. All staff change from their normal clothes into scrubs before they begin work for the day and, when carrying out examination of exhibits, they also wear a laboratory coat, two pairs of gloves, hair net and face mask. The building is frequently monitored to check there is no background contamination and, when any item is examined, quality control samples are taken from the examination bench, exhibit packaging and examiner’s gloves and lab coat. These measures are necessary to provide assurance to the Courts that any GSR was genuinely present on the item before it was brought to the laboratory. Although these are time-consuming tasks, they are an essential part of the ISO/IEC:17025:2005 Laboratory Accreditation standard to which LGC Forensics works.

GSR is analysed by scanning electron microscopy, with energy-dispersive X-ray analysis (SEM-EDX). A set of GSR evidence collection stubs will be loaded into the instrument for overnight analysis. The stubs are scanned by the electron beam and the location of particles with a high atomic number is recorded. The analytical software then revisits these particles and performs an elemental analysis, after which the particle is assigned to one of about 35 classes. Most of these classes relate to material commonly found in the environment, such as gold and silver from jewellery; copper, brass and iron from coins and keys and cerium and lanthanum from cigarette lighters. If some of the particles on the stub contain one or more of the three components of lead, barium and antimony, those particles will be flagged up as potential gunshot residue. On the following day, the forensics analyst will check the particles of interest highlighted by the software and print detailed reports for any confirmed as GSR.

The process is time consuming – the examination of a jacket can generate 10-or-more stubs for instrumental analysis, plus quality control samples if GSR is found. Each stub can easily take five or more hours to analyse. Considerable effort is being made to streamline the process. For example, modern silicon drift analysers can perform the chemical analysis several times faster than conventional analysers, whilst improvements to the analytical software and electron microscopes are also increasing sample throughput. The forensic scientist will also ensure, in conjunction with the investigating police officer, that the examination strategy provides the greatest value for money, by targeting those items and tests most likely to give a useful result. In order to do this effectively, the scientist needs to have as much information about the circumstances of the case as possible.

If GSR is found on an item of clothing (or even if it isn’t), the most important part of the process is the interpretation of results. As with any forensic evidence, it is essential to put the findings into an appropriate context to put before a jury and to ensure the evidence is given in a balanced, impartial manner. Often the strongest evidence can be given when there are two alternative scenarios to consider – one put forward by the Prosecution and the alibi given by the defendant.  For example, the defendant may claim that any GSR found on his clothing was a result of his arrest by armed police officers, rather than the Prosecution allegation that he fired the gun used to kill someone. With those two scenarios in mind, the forensic scientist can consider the composition of the GSR found on the suspect, in the recovered gun and in ammunition used by the police. If the GSR on the suspect is similar to that found in the gun, but different to that found in police ammunition, there may be strong support for the Prosecution allegation being correct, rather than the defendant’s alibi. Of course, the converse may also be true – there may be strong support for the defendant’s case, rather than the Prosecution’s.

The forensic scientist also needs to consider many other factors when evaluating the results.  The higher the amount of GSR recovered, the less likely that it has resulted from a chance event, such as the suspect having sat on a contaminated train seat. The sooner a suspect has been arrested after the crime, the more likely it is that GSR will still be present on the suspect’s hands and clothes. If the suspect has legitimate access to firearms, this could provide an explanation for the presence of GSR on his clothing.

GSR evidence is essentially corroborative in nature and is unlikely to solve a case in isolation – not least because a suspect needs to be identified before his clothing can be seized and examined. However, in conjunction with other evidence, whether forensic, CCTV or eyewitness, it can provide compelling evidence that a person has been involved in firearms crime.