Is forensic evidence infallible?

In everyday life people have grown used to the idea that science and technology are more reliable than humans are. But this does not mean that forensic evidence is always reliable. When people are gathering evidence and interpreting it, the way in which they gather that evidence and the interpretations they make can be just as vulnerable to human error as the evidence of an eyewitness.

It is therefore essential not to trust the evidence just because it is ‘forensic evidence'. Forensic does not mean infallible.

Cell Site Evidence in Murder Cases

If the police have a suspect's mobile phone or even just its sim card they can obtain information from the mobile service provider. This can include data about where the phone has been used or connected to the network at a specific time. This will often be used to try to show the suspect's movements. Sometimes even the lack of mobile phone use can be significant. In one case in which Mary Monson Solicitors was instructed the prosecution were able to show that the phone of one defendant was turned off for the first time in weeks as the phone moved into the area where the murder was about to be committed and turned on again as it moved away from the area just after the killing.

However, no cell site evidence should be taken at face value. All the material gathered by the prosecution (the raw data) should be sought by the defence and analysed by a defence appointed expert. The prosecution ‘cell site report' showing locations of the phone at different times often supports what the police and CPS say occurred, but also fails to mention things which undermine or contradict it. For example, there are often occasions where a mobile phone appears to show a connection with an area, when it is clear that it could not because it is showing another connection elsewhere.

The defence expert should obtain a cell site map and perform a physical survey of the route or area in question. Doing this survey in person is necessary because a mobile phone signal can connect with a number of cell site antennae over a large area due to deflection by buildings or atmospheric conditions. The point is that cell site evidence is seldom as powerful as it seems at first blush if reinvestigated from scratch by the defence.

It is also wrong to suggest that the users of two mobile phones appearing to draw signal from the same mobile phone mast within minutes of each other must be together. Allowance must be made for the difference in time between contacts with the mast, and whether it is possible that they may have moved. The range of the type of mast is also very important. In some circumstances, two phones using the same mast could be hundreds of metres, or even several kilometres apart.

Phone records - calls and texts

When the police seize mobile phones, they will often try to interpret the way the phones have been used to call and text to prove certain aspects of the prosecution case.

One example of the way in which this happens is that the prosecution ask the jury to infer from timing and regularity of calls between defendants that they have been acting together in planning, carrying out, or cleaning up the killing. This is particularly relevant in joint enterprise murder or conspiracy to murder cases. But assuming that just because one defendant or suspect is in contact with another defendant or suspect then that must mean that both are carrying out or planning a murder together is a dangerous thing to do. A persuasive defence advocate, ideally armed with a possible explanation by his or her client, should be expected to show the jury that association is not the same as guilt.

Text messages can be problematic, but the prosecution can often be over-imaginative in their interpretation of the meaning of messages. Street slang or ‘in jokes' can be taken to mean something sinister or meaningful when in fact the explanation is much more boring. Where the prosecution try to overstate their case in this way, an engaging defence advocate should usually be able to win over the jury on these points.

One issue that needs to be carefully considered is whether it can be proven beyond doubt that the mobile phone actually belonged to the defendant, and whether anyone else had access to it. If the handset was not taken directly from the defendant, the prosecution may try to use the list of people that person has called to attribute the phone to him or her by implication. This in itself is guesswork, and murder solicitors and juries should be wary of it. On another note, if it appears that the phone can be attributed to that person, do the defender need to accept that nobody else had access to the phone, or that the phone may have travelled with somebody else, perhaps left in a car? Possibilities of other explanations must be considered with the client if he or she has any.

CCTV Footage

CCTV footage is often used in murder cases to show that the suspect was in a given place at a given time. The vast majority of the CCTV footage which is relied upon is from private sources, such as shops, pubs, garage forecourts etc. CCTV cameras in the public domain such as the local authority or The Highways Agency can be expected to be subject to routine maintenance.This becomes important where the CCTV material is time crucial and the camera has an internal clock showing when footage was supposedly captured. Very often the clocks are not adjusted to GMT/BST or upon examination can be shown to be malfunctioning. So in a recent case in which Mary Monson Solicitors defended, footage which showed a group of youths passing by on the way to commit a murder at a particular time was discredited when a proper inspection by the defence showed the internal clock was inaccurate. As with much forensic evidence, nothing should be accepted at face value.

Vehicle Number Plate Recognition or ANPR in murder cases

Automatic numberplate recognition or ANPR is often used by the prosecution to try to prove the suspect's presence at a certain place, or disprove it. In a recent case defended by Mary Monson Solicitors the prosecution tried to use the absence of ANPR readings on major routes as proof that the deceased had never visited another town, in the defence's case, to see an unknown person who could have been her killer.

The defence were able to show through expert evidence and cross examination that it was perfectly possible and even preferable to make the journey by back roads that had no CCTV coverage. The defence also cross-examined the police ANPR witness, who then accepted the following limitations of ANPR evidence.

  • There is a high rate of ‘misreads' of number plates due to various factors including weather conditions, technical malfunctions, system reboots, and blockage from other vehicles.

  • No statistics are gathered as to the proportion of misreads as against accurate readings.

Computer Evidence

The police will often seize a suspect's computer and may use software to uncover incriminating evidence of searches for weapons, information about murder or chemicals. Where exactly on the computer these pieces of data are stored and how they got there are very important things to consider. In many cases the prosecution computer expert may be forced to accept that the analysis of a computer is often like mining into a mountain of data and that it is usually possible to pull out something that appeared relevant to the prosecution case. It is also now generally accepted that it is possible for material to get onto a computer by way of viruses and malware such as trojans without the owner of the computer knowing. Mary Monson Solicitors recently sourced an expert in the USA who gave evidence that criminal gangs often operating from Russia used domestic laptop computers to store data remotely and without the owner's knowledge. This case had a far reaching impact on the forensic

DNA in Murder Cases

One of the biggest developments in recent years in forensic evidence has been the discovery and use of DNA profiling techniques as evidence of presence, and contact of suspects with a crime scene or with the deceased in murder cases. Samples of any blood, skin or semen found at the scene can be used to build a genetic profile of a person which may correspond to the suspect.

Various methods are used, but in the UK the main one involves a fairly cautious method of matching a large part of the DNA strand of the sample and the suspect, which is generally regarded to provide a likelihood of 1 in a billion people sharing that DNA profile.

Contamination, however, remains a significant issue, and the risk of the results being distorted is much greater as a result of mistakes made in the gathering of the DNA, its handling between the crime scene and the lab, and in the lab itself during testing.

Although control samples are taken to try to avoid contamination taking place, the reality is that the opportunities for contamination are rife, and in cases where identity is challenged, the evidence chain and segregation of materials in the laboratory should be considered by the defence very closely.

Low Copy DNA in Murder Cases

Possibilities of contamination and its potentially disastrous effects are more relevant in cases where so called ‘Low Copy Number' profiling is used. The Low Copy technique is used where a very small sample of DNA is available and this is amplified to create a larger sample. It has been known for such amplified results to be made up of the DNA of more than one person, and even partly contaminated by chemicals used in the amplification process. So great are the problems that this technique was heavily criticised during the Omagh Bombing case. After some indecision, the CPS has returned to the use of Low Copy DNA in criminal cases, but murder solicitors must understand that Low Copy is always vulnerable to attack as inherently unsafe.

Partial DNA Matches in Murder Cases

In some circumstances, only a partial DNA profile can be obtained at the scene. While the police and prosecutors may attempt to use this as evidence against a defendant in a murder case, the defence may have a strong challenge against this. A judge should to give special guidance to the jury about how they should consider this weaker DNA evidence.

Fingerprint Evidence in Murder Cases

Fingerprint evidence has been in use in UK courts for over a hundred years. It is often said that no two fingerprints have ever been found to be identical. But that ‘fact' and the strength of fingerprint evidence in courts is heavily reliant on the human element. This human element is the role of the fingerprint examiner.

The fingerprint examiner's role is to compare the characteristics the sample of the fingerprint taken at the police station from the suspect and the recovered ‘lift' taken at the crime scene. This comparison is a subjective process, and can be highly subject to human error. A 1995 study under the supervision of the International Association for Identification looking at how well expert fingerprint examiners actually performed showed a failure rate of 1 in 5. Experts can also be influenced by the circumstances of their instruction by police officers, who may give them information or an opinion which prejudices their own findings without them realising. The same fingerprints presented to experts with a different back story have been shown to result in different ID results.

The fact that ID evidence can be so prone to human error means that in a murder case, the solicitors must be very careful not to accept it unless it is in keeping with the client's case, and if it is not, they should instruct an independent fingerprint expert to look at the evidence afresh for a second opinion.

Firearms Evidence

Tracing the bullet to the gun

If the murder weapon is a gun, then any bullets fired from that gun will invariably carry similar markings or ‘striations' as a result of imperfections inside the barrel of the gun. This means that if a weapon and bullet are both found then it can be usually shown using a specialist microscope whether the bullet was fired from the gun. Problems can occur if the bullet shatters into pieces. It may still be possible to know the calibre or make and model of the firearm in these circumstances. A national database of the identities, characteristics and histories of individual firearms which have been used in crimes is maintained by the National Ballistics Intelligence Service, or NABIS. This includes profiles of so-called ‘inferred firearms' for which there is data of their history but the weapon has never been recovered. The prosecution may try to show that the location or circumstances of the gun being used on other occasions is significant evidence against the defendant in the current case.

Gunpowder residue and burns

When a gun fires, it can leave gunshot residue (or GSR) and also possibly minor burns in the immediate vicinity. This can leave evidence on both the assailant, the targeted person, any bystanders, and even on nearby objects. This evidence can be used to try show the proximity of the assailant, the direction of the shot, and in some circumstances even the type or model of weapon.

Gunshot residue evidence has limitations, however. Cross contamination can be an issue, and the normal behaviour of gunpowder being discharged can be affected by atmospheric conditions such as wind, temperature and humidity. It is far from exact when trying to show direction or estimate exact distances.

Fingerprints on bullet casings

Acid from a fingerprint on a bullet casing imprinted during loading can melt into a permanent fingerprint when the gun is fired. The fingerprint is often only partial, so this evidence can be inconclusive to show identity.

Bullet holes - trajectory

It can be possible using the bullet holes in fixed objects such as walls or in a body to work out certain information about how bullet or bullets were fired, such as the possible position of the person firing, and possibly also height. However, what the police or prosecution say about these calculations should never be taken for granted. Showing the trajectory of a bullet is not the same as physically recreating the moment of a shot. Whether the firer was sitting or standing, holding the firearm in a high or low position, are all factors which can affect any interpretation that the prosecution may want to make.

If you or a family member are worried about an investigation or prosecution for murder or manslaughter, please call us and speak to one of our specialist murder solicitors throughout the UK. We are always happy to talk.


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