Question: How do you find out who actually did somthing like a murded.ect with finger prints? ~doggy50k~

Mark Hill answered on 19 Mar 2011:

Hello doggy50k,

This is a really interesting question, which really needs more space to answer properly that I have here.

Excuse me if I explain matters on a basic level than you are used to, but I don’t know your age group, or your level of knowledge.

The uniqueness of the fingerprint, toe print, palm and foot print allow for comparison examination of marks found at a crime scene (or collision scene, on vehicles), and the prints that are held on record, which have been taken from convicted people.

I must add here that elimination prints are often taken from people who may have been near, or at, a scene, in order to separate their prints from the offender’s. There are strict rules controlling the use of elimination prints.

Prints can be in any number of substances – blood, grease, oil, paint, sticky tape, blue tack (other plastic adhesive materials are also available) and more.

The more common print is essentially made up of sweat and eccrine, apocrine and sebaceous secretions from the pores that are in the grooves between the ridges of a print. So a print is really a ‘negative’ image of the finger, toe, foot or palm that created it.

From the examination of a scene, identification of prints, whether visible, or latent, those prints are developed, using a metal flake, or fine powder. Sometimes other chemical treatment may be used on different materials, such as ninhydrin on paper, super glue fumes (cyanoacrylate) on other surfaces that cannot be treated by other means.

The marks collected are then compared with known records of marks with similar patterns. There are several general patterns that can be readily identified in prints, such as arches, loops, whorls, bifurcations, deltas. Once the general pattern features are identified, then similar print records are compared. Gradually, by refining the similar patterns, the number of possible matches are narrowed.

When a very similar print is found then the expert will decide whether there is a match or not. Until about ten years ago it was necessary to find at least 16 points in common between the scene print and the comparison print. However, it is now down to the expert to make a judgement. There are now also computerised scanning and comparison devices in use, but it still finally rests the expert to present their opinion.

The National Fingerprint Bureau holds well over 5 000 000 named prints on record and some 500 000 prints from crime scenes, which have not as yet been matched.

Really, if a person has not been convicted of a crime and their prints taken, then we are not going to match their prints with those from a scene.

Once a print is matched to a person, they are then interviewed and asked to explain how their print was found at the murder scene. It may then be for a Crown Court jury to decide whether the account the suspect gives is the truth, or a lie.

Finally, there is now another way of identifying a suspect from a print, or rather the biological material from a print – DNA. There may be sufficient traces within the print for analysis, again against records held. Sorry, but that would be for another question.

I hope that this answers your question. I am sure that Niamh and Jodie can add more detail, but this is based on practices that I have and still use.

Thank you for an interesting and, so far, original question.

Mark.