The fibre that nailed the murderer

A scene from CSI (© CBS).

There is nothing like that. I am referring to the title of this article. Despite what happens in fiction, where often the resolution of a case depends on the discovery of a fibre, typically a single and isolated one, found at the scene and on the victim (it often happens to Sara Sidle from “CSI”; see photo), in reality such a fibre would serve to demonstrate just nothing at all.

It’s clear that certain materials release tiny fibres, which, if desired, can be compared, for example, with the clothes of a suspect or with some other textile object that belongs to the latter.

Let’s completely leave out the usual imaginary databases, like the one, in one of the latest episodes of “Bones” I recently watched, that linked a synthetic fibre to the mat of a luxurious car, which, as usual, was a limited series, allowing for confirmation of the suspect. However, even assuming that the database exists (I don’t believe so), I have strong doubts that a particular fibre is only used for the mat of a car model (we know they do them all in China, often in the same factory from which those for the cheapest small cars come).

Instead, let’s get back to the case where there’s already a suspect and you can make a comparison. But even if this comparison is positive, what would it demonstrate? Nothing.

Identical fibres are found in very different materials, moreover our fibre could be there because of an innocent contamination preceding the murder or because it was transferred to the victim after a previous contact with someone totally unconnected with the facts.

For this reason, fibres are only supporting evidence that therefore doesn’t add certainty.

This applies also for hair. In this case a comparison may be more useful, as it helps to narrow the field. If a long blond hair is wrapped on the murder weapon, for instance, and the suspect has long blond hair of the same colour and similar characteristics (such as size or the fact that the colour is or isn’t natural), there is a certain probability that it belongs to that person, but probability is not certainty and this makes it just supporting evidence.

The real breakthrough would be achieved if the hair in question still had the hair bulb from where you can collect and then analyse DNA. If there weren’t a good reason, independent from the murder, able to explain the presence of the hair on the weapon, the suspect would be in big trouble.

Another kind of evidence that can be found at the scene is a shoe print, maybe a bloody one. If it’s different from that of the victim, almost certainly that shoe print belongs to the killer or other person who was with him at the time of the murder.

The shoe print can allow criminologists to trace back to the shoe size and, if it shows a particular design, even the brand (especially if that famous database of shoe soles existed!). Once again it can be very useful for a comparison. The problem remains that, if it isn’t from unique handmade shoes (quite rare nowadays), this type of evidence does not give certainty, unless you find a specific correspondence concerning the wear of the sole, due to the unique way that each of us has to walk and wear out some of its areas. In this case, the time factor becomes crucial, because the wear and tear continue, if the murderer keeps on wearing those shoes. You have to make the comparison within days, at most a few weeks, otherwise you won’t find any matches.

Like shoe prints, there are other types of characteristic signs. Let’s consider a screwdriver used to force a window. It certainly leaves a mark on the frame, which corresponds to its shape. If the screwdriver isn’t new, it’s become worn, then the mark it leaves behind is unique. As for the shoes, and for the same reason, the comparison, however, must be done in the shortest time possible.

What if the screwdriver was the murder weapon?

Certainly it left a mark on the body of the victim, maybe not as distinguishable as on the frame, unless it has affected a bone. Finding a similar weapon at the suspect’s house and detecting traces of blood on it (which, as we all know, is very difficult to remove, so much to stay there even if you don’t see it with your naked eye) could be an overwhelming proof. Even more if there are his fingerprints on it and he doesn’t have an alibi for the time of the crime. Unless someone has decided to frame him, using his screwdriver, which happens more often in fiction than in reality.

A scene from CSI: NY (© CBS).

To discover the murder weapon we have often seen criminologists in the TV series get to pierce gelatine men or carcasses of animals with objects found in the house of the suspect. Usually this wasteful practice leads nowhere, except the rare case that the suspect was framed.

The real culprit, of course, gets rid of the crime weapon!

Even more expensive is the practice of raging on surrogates with random objects that the protagonists of CSI simply place on a table and try one by one. The aim is at least to understand which weapon they must look for. The possibility of catching the right one among an almost infinite number of objects that tear the meat in a similar manner is close to zero. Well, in this case, instead, immediately a strange tool comes up (what luck!) that perfectly corresponds to the shape of the wound. Usually, when it happens, it provides valuable information, since a suspect uses something like this for work or hobbies.

And then there are the famous traumas from a blunt object.

The victim was hit in the head by an object that might be a hammer, a mace, a lamp, a trophy or something. The impact caused a peri-mortem trauma (sometimes not immediately visible, but it appears in the form of haematoma after the body is kept in the freezer for a few hours) and maybe left an evident mark on the skull bones.

And so the criminologists, with great fun, put on their overalls and goggles (that outfit that they do not use at the crime scene; see photo from “CSI: NY”) and begin to hit the poor dummies, until they find the right object.

The mechanism is identical to the case of the screwdriver: if the weapon is found at the suspect’s home, someone has framed him; if it isn’t found, however, they will be able to locate it by taking chances amongst thousands of possibilities, shamelessly wasting puppets, gelatine and pig carcasses, and from there they’ll unveil the unthinkable culprit that uses the same type of tool to make some innocent do-it-yourself. Then, by analysing his toolbox, they will find a bar with the same profile, which will be bloody or recently cleaned with bleach (unlike other tools that are filthy).

I could go on like this forever telling you about all these proofs that are the daily bread of criminologists in fiction and whose discovery is, consequently, the basis of the plots of crime stories focused on forensic science.

As an author I use several of them as well, especially since I know that’s what the reader would expect. And then, let’s face it, it’s fun to use them! But I prefer to highlight how their usefulness is limited, often exploiting them as elements that support the investigation process made of intuition and imagination, or that exclude certain scenarios. This is because this evidence is often most useful to exclude than to confirm.

For instance, in “Syndrome”, an interesting element when analysing the scene of two murders is the total absence of shoe prints in a nearly immaculate flat, where a dirty corpse lies whose soles have some soil. This finding brings Detective Shaw and his team to conclude that the murderer has deleted these prints or, even better, that the victim has never walked on that floor, where he was just dumped, therefore that isn’t the primary scene of the crime.

In “The Mentor”, instead, I exploit the case of a haematoma that appears later (because it isn’t visible on the corpse immediately after death) and reveals that the victim was pushed with a shoe whose pointed profile reminds the footwear of a woman. This leads investigators to speculate that a woman is the killer (or the killer’s accomplice) and pushes Detective Leroux to question an eyewitness, who maybe saw someone leaving the crime scene.

The beauty of it is that I didn’t learn much of this by reading treatises on forensic science, but reading the novels by Patricia Cornwell, watching “CSI”, “Bones”, “NCIS”, “Body of Proof” and many others. All of which, taking a cue from science to create stories, in addition to entertain people (as they are entertainment tools), somehow enrich the latter, leaving them with a little more knowledge and, at the same time, with a certain curiosity to learn more.

Originally published at on October 12, 2016.