Collette McKay, fingerprint expert

I WORK as a reporting fingerprint examiner with the Scottish Police Authority based at the Scottish Crime Campus in Gartcosh. My job involves analysing, comparing and identifying fingerprints. That could be from a crime scene, elimination prints taken from a house that has been broken into or the identification of deceased persons if we hold those fingerprints on file.

I've been doing this job for 20 years. I started off in 1998 as a temporary clerical officer. It is a great job and I find it interesting. In Scotland, there's less than 50 fingerprint experts. You can't simply walk in off the street and become a reporting fingerprint examiner. It involves three-and-a-half years of on-the-job training where you are constantly honing your skills and undergoing continual assessment.

There are three other fingerprint units located in Edinburgh, Dundee and Aberdeen. All the units in Scotland are UKAS-accredited for their examination processes and procedures. That is important because it puts fingerprints on the same level standing as all the other scientific disciplines within forensics.

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Fingerprints are unique. It has never been known for two people to have the same fingerprints. This is down to how they are formed in the womb. Cells fuse together randomly, and this is where the friction ridges develop on the fingers. That happens when the baby touches the inside of the womb with their hands and feet. The detail within those patterns will never be the same. Not even identical twins make all the same movements, touching patterns or apply the same pressure when they are in the womb.

We used to have a 16-point standard for fingerprint identification but that was done away with years ago because there was no scientific basis for it. When analysing a crime scene mark, we consider everything that is within that mark, including the background it is on and what has been used to develop it. Then we go further in and look at the pattern and characteristics.

It is not my job to say whether a person is guilty or innocent. I can't go into court and offer my opinion on the how, when or why they were there. All I can say is that the person has touched an article at some point and the fingerprints left behind were identified as theirs.

On TV shows like CSI: Crime Scene Investigation there will often be a screen showing the fingerprint that suddenly starts flashing "Match! Match! Match!". That's not how it works. We have an automated fingerprint database, but the machine is simply one of many tools we use. If it comes up with a hit, we still always carry out a manual comparison.

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Every case we work on – whether it is negative, identified or insufficient – is then verified by another fingerprint expert. It is never just one person who looks at it. We always make sure a mark has been looked at independently by another expert who then does their own analysis, verification and conclusion.