Sign in. It’s quick, free and it’s up to you.
An account is an optional way to support the work we do. Find out more.
Sponsored
Could you solve a murder? Here’s a look at the science of a crime scene
From blood stains to a person’s Whatsapp history – here’s how crimes are solved.
DURING THE INFAMOUS trial of OJ Simpson, it emerged that a vial of the former NFL star’s blood had been stored for a time in the pocket of one of the police detectives, who then visited Simpson’s home – calling into question how the evidence was handled. Almost 25 years later, the chain of custody (when, where and with whom evidence is stored) is highly regulated, and technologies such as digital forensics and Y-STR DNA profiling are now enabling prosecutors to link perpetrators to their crimes. So, what can science tell us about the evidence that clings to crime scenes? We spoke to Dr Martina McBride, forensic scientist at Forensic Science Ireland (the official lab of the Department of Justice).
What kind of clues can blood hold?
Blood can be crucial for a garda investigation not only in terms of what it tells us about the victim, but also in terms of its ability to link a suspect to a crime scene, says McBride:
If the attack involves frenzied knife crime, the perpetrator will often cut themselves – so we will look for blood that isn’t the victim’s. This is done through blood stain pattern analysis. Drops at 90 degrees will appear circular and can indicate someone was walking and bleeding.
Equally, investigators should always look at the entry and exit points for biological evidence that may lead us to the person who committed the crime, shares McBride:
If they have smashed a window and cut themselves and touched door or window handles, we will take blood samples by using a swab, adhesive tape or by scraping it into a vial. We then subject it to DNA profiling, to see whether it matches the victim or the perpetrator.
What can microscopic fibres tell us about the crime?
As brilliant as biological evidence is for linking particular people to a crime, it’s the fibres that they leave behind that can actually tell you when it happened, says McBride:
Fibre evidence can be really important as it can give you a timeline – if there’s still fibres present it will be a recent contact. With a blood stain you don’t know how long it’s there. If it’s a murder scene you can pin it to that, but say if there’s a history of domestic violence, you can’t age that blood.
Fibres can also be left behind by a perpetrator at entry or exit points to a crime scene, says McBride:
If they entered through a window, they may leave fibres of their clothing on the frame, or we may find fibres of the victim’s clothing on their clothing. With this, we will take the garment, use sellotape to take samples, put these under the microscope, pick off target fibres and test them for things like dyes and fabric type.
What can we learn from the weapon that was used?
Just like blood found at the scene, a murder weapon can hold a lot of information about who may have committed the crime, states McBride:
If the knife is found away from the scene, we want to look for fibres that may be from the clothing of the injured party, which are left behind as the knife enters their body. We will also look for blood, touch DNA (skin cells and sweat) and fingerprints.
John Keary, coordinator of the Forensic Science programme at Galway-Mayo Institute of Technology explains how these fingerprints are taken:
Gardaí would use a white fingerprint powder, the print would be lifted if it was latent (invisible) using a photograph. They would then scan it in and use an automated fingerprint system to see if they have the person on file from their fingerprint database.
Keary also details that particles left behind from gun crime can leave vital clues:
Every time you shoot a gun, you generate gunshot residue and leave lead particles from the bullets – leaving an elevated level of lead on your hands and the sleeves of your clothes. Also, as a bullet rotates down a gun, it gains particular markings and these can be matched to a particular gun by the Garda Ballistics Section.
What kind of evidence is most likely to lead to a conviction?
Is there a particular type of biological evidence that is more likely to convict a suspect? Some types of samples are easier to extract DNA from than others, says Keary:
Without doubt, DNA is the strongest type of evidence, especially from bodily fluids, blood, saliva and semen. To get good DNA from hair, it must be a root – where the concentration of the DNA cells are. The shaft of hair isn’t as DNA-rich, though it is possible to get a sample.
What about blood? Well, that’s an interesting one, says McBride, who explains the process of identifying DNA: “DNA is present in every cell except red blood cells, so we look at white blood cells.”
What impact does the location have on a crime scene?
Arguably, if a body is found in winter, the evidence investigators are able to collect can be of a higher quality than if it is found in summer or on a wet day, says McBride:
We have had instances where we got a DNA profile from semen from bodies that were outside for a few weeks. Obviously any biological substance will rot in heat or wet but it tends to last longer in frozen blood and semen.
As Keary details, the preservation of the crime scene to avoid contamination can be crucial:
If it’s in a public place it will be sealed off for a few days so there’s no contamination of evidence, which can make it useless at analysis stage. Investigators would be gowned up with gloves and hairnets so that there is no chance of their DNA ending up on the samples.
Are crime scene investigations anything like the movies?
Shows like CSI and Mindhunter can be fascinating, but the reality of evidence analysis shown in them isn’t always so quick, says McBride:
[On TV], you get everything very fast – usually within half an hour. The reality is that techniques such as fibre analysis take a long time. There’s a lot of searching involved – you need to pick off fibres from each individual area on a garment and tests each. It’s very manpower-intensive and a lot of fibres aren’t relevant.
Similarly, it doesn’t always end up in a satisfying ‘case closed’ scenario, says Keary:
There’s a misconception that everything gets solved, which isn’t always the case. We see very few unsolved cases as it wouldn’t make good TV.
What kind of evidence will the victim’s phone contain?
There’s an area of forensics that is becoming increasingly more important in trials, says Keary – and it was a crucial part of two high-profile Irish cases – those of Graham Dwyer and Joe O’Reilly:
Digital forensics is becoming crucial. Everyone leaves a footprint of emails, messages and phone records (even if they’re deleted) and its importance can be underestimated. In both the Graham Dwyer and the O’Reilly case, phone records were able to pinpoint the perpetrators at particular locations at specific times.
And it’s not the only part of forensics that has benefited hugely from advances in technology, DNA profiling is constantly evolving, says McBride:
DNA processes are becoming more rapid all the time – we can now get a partial profile within three hours. Y-STR DNA profiling can also be very useful – it’s used when there is a lot of mixed DNA, for example in a vaginal swab. It allows you to pull out the male DNA, meaning we can get profiles from materials that we couldn’t have in the past.
What else should we consider about evidence?
No matter what the science says, you also need to consider the context of each part of evidence, says McBride:
Fibre contact between people who have legitimate contact won’t mean anything but it will be very important if they don’t. Equally, blood stains won’t be as important for someone who found the body.
Think you could assess a crime scene armed with that scientific knowledge? For Science Week 2017, Science Foundation Ireland is calling on the general public to #StopAndAsk questions about the world around them. In turn, the scientific community will answer some of these questions on social media, facilitating conversations between them and the general public, using the hashtag #StopAndAsk. Want to know more about forensics and the world around us? Science Week takes place from the 12th to the 19th of November and includes events with Dara O’Briain, umami burgers, bog bodies and theatrical performances, with hundreds of events taking place nationwide.
have your say