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WHEN MURDER INVESTIGATIONS GO WRONG – A Defence Lawyer’s perspective – part 1

The unprecedented number of murders in London have led to much comment about the cause of such violence on our street (see Caroline Liggins comment). However, we should also consider the strain and pressure that this spree will place on the criminal justice system.

Having defended in a number of murders in the capital and beyond, Raj Chada, in a series of articles looks at the issues that arise in murder investigations, how it can go wrong and the need for an active defence with forensic scrutiny. In the first article, Raj looks at DNA evidence….

The mere presence of DNA does not prove a crime happened or that there was a particular participant to that crime. The DNA does not tell you how or when the material got to its discovered location. Contrary to Hollywood crime show (and at times UK Prosecutors) oversimplifications, DNA is not a synonym for “guilty.”

Make no mistake though, in straightforward cases, DNA evidence is regarded as the “gold standard” in forensics work. This is because forensics can compare the DNA of a suspect with the DNA recovered at the scene and produce a statistical analysis of whether the samples “match”.

However things are much more complicated when DNA comes from a small amount of material (low trace DNA) or where there is DNA of multiple individuals.

Despite Court of Appeal decisions1 that say that these forensic reports are admissible, there are still many issues that Defence lawyers have to grapple with.

Firstly, there is an issue of whether DNA could have been “transferred” to the scene. This has been particularly highlighted by scientists such as Professor Jamieson as a risk in low trace DNA cases2.

This issue has also been explored extensively by Greg Hampkian in the US. Hampkian is a researcher at Boise State University and head of the Idaho Innocence Project. He produced reports for the defence in the notorious Amanda Knox case in Italy that eventually led to her conviction being overturned.

In one reported study3, Hampkian conducted an interesting experiment with his students. They collected five soda cans from the University Dean’s Office. They put the cans in individual evidence bags. Then, without changing gloves, they put five newly bought knives into separate evidence bags. They found DNA from a member of the Dean’s staff on one of the blades – yet that person had not touched nor even been in the same room with the knives. The risk of DNA transfer or contamination in low trace DNA cases would appear to be real.

DNA analysis can become even trickier when a mix of DNA from various individuals are found in a single crime scene, With a simple sample, analysts look at two sets of peaks at a given locus; one for the victim and one for the perpetrator. With mixtures, they are looking at a mix of peaks, with no indications of which pairs go together, or which source they come from – aside from those known victim. At that point, the analysis becomes highly subjective.

Another US expert, Michael Coble, National Institute of Standards and Technology in Gaithersburg, in Maryland, US set up a hypothetical scenario in which a mix of DNA from several people had been found on a ski mask left at a crime scene after a series of robberies. Coble asked 108 labs across the US to determine whether a separate DNA sample, which he posed had come from a suspect in the robberies was also part of the mix. 73 labs got it wrong, saying that the suspect’s DNA was part of the mix, when, in fact it was not4.

There is a risk in murder investigation that DNA is regarded as the problem solver and that no further investigation is required. Indeed it risks lazy prosecution and demands active defence. Human error, contamination and transfer all must be explored.

A thorough analysis of the prosecution’s forensic report is also required. It should be noted that the in the cases already cited such as R v Dlugosz5 , the Court of Appeal accepted that DNA could be admissible – even where due to a “mixed sample”, the expert could not say, in statistical analysis exercise that the DNA was attributable to an individual. Instead, due to the mixed sample, an expert could only provide a subjective opinion based on his/her experience. This is a far cry from the “gold standard” that DNA analysis began with.

Again, the US is perhaps already further ahead of us in recognising the dangers. The President’s Council of Advisors on Science and Technology (PCAST) is an advisory group of leading scientists and engineers, appointed by the President of the US. In September 2016 PCAST released a critique of several methods used in forensic science – including in mixed DNA samples6.

The PCAST report said that “expert witnesses have often overstated the probative value of their evidence, going far beyond what the relevant science can justify”. Further that “subjective methods require particularly careful scrutiny because their heavy reliance on human judgement means that they are especially vulnerable to human error, inconsistency across examiners and cognitive bias….”

There is a specific concern raised about the interpretation of mixed samples DNA, even where software programs are used to assist7.

As with much else in murder cases, DNA analysis require careful and forensic scrutiny – it should never be accepted as a given.


R v Reed & Anor [2009] EWCA Crim 2698, R v Dlugosz [2013] EWCA Crim 2

See comments by Professor Jamieson in Daily Telegraph article by William Lanley dated 6 March 2012

Science Magazine – Forensics gone wrong : When DNA snares the innocent 7 March 2016

4 Reported in Science Magazine – Forensics gone wrong, as above

R v Dlugosz [2013] EWCA Crim 2

Presidents Council of Advisors on Science and Technlogy. Report to Forensic Science in Criminal Courts : Ensuring Scientific Validity of Feature-Comparison Methods. September 2016.

Also referred to in The Barrister Magazine – Its Science Jim but not as we know it…oh no its not science after all – Dr Scott Bader and Professor Allan Jamieson of the Forensic Institute – 21 February 2017