Science Interviews


Tue, 3rd May 2016

Caught red handed: who shot the gun?

James French, University College London

Listen Now    Download as mp3 from the show Can Science Prove Whodunnit?

Forensic science is improving, but does this mean we’re more likely to catch the rightGun crook? While we are getting better at detecting clues, this doesn't mean we're any better at interpreting them, which could lead to a miscarriage of justice. James French is from University College London’s Centre for Forensic Sciences, and he spoke to Georgia Mills about why there's a problem with our understanding of the field...

James - There are a number of issues that have arisen in recent years in forensic science. It’s been highlighted that, in a number of fields of forensic science, there’s a lack of underpinning research to underpin our opinions and conclusions. This was captured in the report by the National Academy of Sciences in the US in 2009. I mean they really criticised forensic science as having a lack of scientific basis in a number of different areas and, I guess, the work that we try to do at the Centre tries to correct that to some extent and to improve the scientific basis for drawing conclusions in forensic science.

Georgia - But surely all of our technology is getting better so our conclusions should be getting better too?

James - I think this is actually one of the problems. We’ve come on so far in different areas of analysis and forensic science but there are still many unanswered questions and gaps in our knowledge in terms of interpretation. So, while we were able to answer the sort of “what” questions with greater certainty, the sort of “how” and “how did it get there” and “how long has it been there,” they’re still questions we need to engage in more research in order to understand.

Georgia - So what specifically do you work on?

James - I’ve conducted some research into looking at gunshot residue. Gunshot residues are produced when a firearm is discharged. They’re made up of different compounds from the bullet itself and under high temperature and pressure, these particles are formed, they cool and condense as they are ejected from the firearm, and they’re deposited in the vicinity. We might look to recover them at the scene of an incident involving a firearm and they can provide useful intelligence and evidence in the investigation of crime. By analysing the gunshot residues we might be able to determine something about the ammunition that was used, but also the presence of material on a suspect might indicate that they’ve been involved in the incident. So it could be highly valuable information. But, as I mentioned before, there are a number of gaps in our understanding when it comes to gunshot residues.

Georgia - We’ve got a demo here. We didn’t get hold of a real gun, I’m sure you’ll be glad to hear, but I’ve brought a toy gun that’s used for shooting flies...

James - Okay.

Georgia - So, can you show me what would happen in terms of how the gun would leave residue on your hands and how you might go about finding it?

James - So, I’ll use a little bit of artistic licence here. So we can imagine that this is just a handgun.  So, if I was to hold it like I’m showing you now, which is basically holding it in my right hand while supporting with my left hand and I was to fire the gun, the gunshot residues would be part of the blast cloud that was ejected at the front of the gun but also a little ejection ports in the side. This would form the cloud of residues that you see in a slow-mo picture of a firearm discharged and, typically, that cloud would be propelled back towards the shooter and in the vicinity.  And residues we would expect to be deposited on the hands, particularly on the back of the hands of the shooter, also on their face and hair, as well as their arms and sleeves but also around and into the environment surround the firearm. So, if I was to discharge the firearm as shown…

Georgia - Got for it…

James - Okay. We would expect to recover gunshot residues, potentially, from the back of my hand that was used to fire the gun. So, I’m going to sort of seed my hands with some fake gunshot residue, which is just a UV powder. I’m putting it particularly on the area that we would expect to locate it in and that’s really between the thumb and index finger and in the gaps between my fingers there. And we can just see that actually that reflects under UV light…

Georgia - Shining blue thumb now.

James - Blue, purple. So when it came to sampling, imagine I was a suspect  and I’ve been apprehended, I might be sampled for gunshot residues, and the way of doing this is really quite simple. We just use a little sticky self-adhesive tab that’s attached to a stub and these are sealed in a sealed tube that ensures that there can be no contamination from the environment. And the process is simply that with gloved hands, we would look to dab on the back of the hands, particularly focusing on the sort of webbing between the thumb and index finger to ensure that any residues were collected. Also in the cracks between fingers and knuckles as well to ensure that we were collecting the maximum amount of material. And that would be quite standard procedure after a firearm is discharged.

Georgia - Meaning you would be caught red, or in our case shining purple, handed - with evidence that a gun was fired in your hands. But does detecting this residue really mean it was you who dunnit? Part two of our demo suggested not…

James - So, if I was to shake your hand in the normal way and then we can look at your hand using the UV light, you can see that there’s some reflecting between your thumb and forefinger there.

Georgia - Yes. I see, I’ve been contaminated!

James - Yes, and potentially implicated in a crime. You’ve also got some there that I can see with the naked eye as well. So there’s a transfer that's occurred there and we’ve just demonstrated the principle of secondary transfer and it’s been shown that this is applicable to a number of different types of evidence, including gunshot residue and other traces, but also DNA as well.

Georgia - It’s not just fun with toys, James and his team have tested this effect with a proper controlled experiment. They got police to fire real guns, and then to shake hands with people who hadn’t been present at the firing range.

James - The results of those experiments were that we found that gunshot residues could be transferred in fairly significant quantities from person to person. We actually found that the shooter was able to transfer material to another person who was then able to transfer to a third individual. Material was also transferred through the handling of a fired firearm as well. It’s really just an extension of one of the basic principles of forensic science which is Edmon Lochard's axiom that every contact leaves a trace. And the aim of this research was not really just to demonstrate that this kind of thing can occur. But it was to inform forensic scientists who encounter gunshot residue about the possibility of the evidence that they’re observing having arisen from a secondary transfer.


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