Fingerprints are unique and for this reason are considered the most effective method to establish the identity of a person together with the analysis of DNA, which is much more difficult to find at the crime scene. But, unlike DNA which is compared by means of an instrumental procedure giving a response on which, except in case of manipulations of the sample, there are no doubts (even if it only provides a statistic), the examination of fingerprints is instead performed visually by a person. The computer can select some possible profiles that display similarities with the fingerprint found at the scene, but then the criminologist is the one carrying out a visual comparison and making a decision.
We know that the human factor always has a margin of error, yet we have realised that this can be true even in the context of the comparison of fingerprints only when some sensational cases of miscarriages of justice occurred.
One of the most famous concerns the terrorist attack in
in 2004. A plastic bag containing detonators was found
at the scene. A latent fingerprint was detected on it, which was then
digitised and transmitted to Interpol. The automated comparison with the
database of the latter had provided twenty results. Among them there was
one that, according to an FBI fingerprint expert, presented a very high
match, so much to bring them to say they had found the owner of the
fingerprint detected at the scene. This conclusion was later confirmed
by two more experts both from FBI. Madrid
The fingerprint was of a Canadian, called Brandon Mayfield, a Muslim lawyer, married with an Egyptian woman. The man denied having anything to do with the attack, but was still arrested according to the discovery of his fingerprint.
Some time later another independent expert confirmed for the fourth time the conclusion of FBI, but shortly after the Spanish police claimed they had found the real culprit, the owner of that fingerprint (an Algerian, who was later incriminated for the bombing). Because of this, Mayfield, who really had nothing to do with the case, was released.
The question that arises is: how is it that four experts were convinced that the fingerprint found at the scene was his? After considering this, how much can you trust the use of fingerprints to establish the guilt of anyone?
And this case is not even the only one, although it is the most impressive.
Another one, for example, occurred in the
, where a fingerprint was found at the scene of a murder (the victim’s
name was Marion Ross) and was then assigned to a detective (Shirley
McKie) who actually worked at the case. You might think about a typical
example of unintentional contamination of the scene by the police, but the
detective had never entered the bathroom where the fingerprint was detected
and, since she continued to affirm that, she was accused of perjury and lost
her job. UK
Then they realized that the fingerprint wasn’t hers. There had been, in fact, a human error by the criminologist who had performed the comparison. The detective regained her job, but at the same time as the real culprit of the murder had been found (his name was David Asbury) again thanks to a fingerprint, the court did not accept the latter as evidence and he was released.
Again in this case a mistake was made, with the aggravating circumstance that it’d had a further negative consequence: that of making the real culprit be released.
How many more errors like these are made and never revealed? How many people have been exonerated or, worse, incriminated and maybe imprisoned because of these mistakes?
We don’t know, but certainly all this highlights how the use of fingerprints or other physical evidence, whose identification depends largely or exclusively on a decision taken by a human, might not give certainties at all.
Of course you can reduce the margin of error with appropriate precautions.
In Mayfield’s case the error was probably due to the fact that all experts involved knew that the man was a Muslim (which can cause a prejudice) and with McKie the fact that she worked the case (other prejudice), but also that those repeating the comparison after the first one were aware of the previous conclusion of their colleagues (comparisons should be blind).
All this concerns reality, which once again looks much more grudging of certainty than fiction.
I honestly cannot remember a single film, novel or TV series in which the criminologist on duty made a mistake with the identification of a fingerprint’s owner. In fiction these professionals are shown as infallible, and if there is a mistake, it is due to a manipulation of the scene by the offender or is voluntary.
The latter cases include the story of Dexter Morgan, main character of the TV series titled “Dexter” (and the books by Jeff Lindsay from which the former is developed), the forensic haematologist who often alters his conclusions or manipulates evidence during investigations to cover his own actions as serial killer, or to ensure that a certain person isn’t arrested so that he could kill them.
In short, fiction tends to enhance the cunning of the protagonists, and defects, when they exist, never concern their job. Typically the investigator drinks (or even uses drugs), eats little, sleeps little or nothing, and maybe doesn’t even wash himself, but when it comes to investigate a murder he is always very clear headed!
An example of this cliché is the series of books by Michael Connelly starring Detective Harry Bosch, from which in recent years the TV series “Bosch” has been produced by Amazon Studios.
In the second book, “Black Ice”, there is a mistake in the identification of a fingerprint’s owner, but it is something voluntary and orchestrated to distract the investigations and not a simple mistake made by those who made the comparison.
Human error without an ulterior motive doesn’t work so much in fiction, perhaps because it is too realistic.
I must admit that while writing the Detective Eric Shaw Trilogy I haven’t used and don’t intend to use such errors by the main characters. Sometimes they may miss a detail (because this is useful to the plot), but never fail a comparison of a fingerprint. The errors at best are made by the antagonist.
This doesn’t mean that I don’t exploit the human factor. Detective Shaw, in fact, uses this type of vulnerability in forensic science to domesticate evidence, as it happens in the case of Johnson in “The Mentor”, in which Shaw causes that the man’s fingerprints are detected on the murder weapon to induce him to confess.
But what if following his own instincts he would aim to the wrong person? Let’s consider that he is absolutely sure that he had found the culprit and forces the situation by manipulating evidence so that this person cannot escape justice.
What if, some time later, maybe years later, something would turn up that casts doubt on the validity of that belief?
This is the possible doubt that I will explore in the final book in the series, “Beyond the Limit”.