How fingerprint experts can make their mark on your defence case

When shopkeeper Thomas Farrow and his wife Anne were murdered in 1905, a bloody thumbprint found at the scene made this the first British murder to be solved and prosecuted using fingerprint evidence.

This case seemed to herald a new dawn of evidence. One where anyone who left a fingerprint at a crime scene could be caught and prosecuted. However, the reality is somewhat different. As we explore in this in-depth article which covers:

• How fingerprint evidence includes more than fingerprints
• Capturing the fingerprint
• The importance of fingerprint quality
• Analysing fingerprints – opinion not fact
• Opportunities to challenge fingerprint evidence
• What can influence fingerprint analysis outcomes?

More than fingerprints

Did you know that fingerprint evidence can include more than prints of your fingers? The term ‘fingerprint’ actually covers a wide range of areas which includes:

• The entire palm surface from the wrist to the tips of the fingers.
• The pieces of skin between the fingers.
• The friction ridges along the inner surface of the fingers and thumbs (known as phalanges)

The arrangement of friction ridge details, which make up fingerprints, are unique to each individual. They are made from raised areas of skin called ‘ridges’ which are separated by depressed areas called ‘furrows’. Fingerprints remain essentially unchanged throughout life, and no two people have been found to have the same arrangement of friction ridge detail, including identical twins.

Prints from toes and feet can also be used to identify someone. This uniqueness and persistence means these marks provide a useful way to identify someone.

Capturing the fingerprint

Watch any police drama on TV and you’ll have seen the basics of gathering fingerprint evidence. Black carbon powder is dusted onto surfaces, sticking to the sweat left by the friction ridges of the fingerprint. These marks are then photographed or lifted using DCF tape so they can be stored as evidence.

If the prints are on a movable item, they can be developed using specialist fingerprint enhancement equipment and techniques. The context in which a fingerprint is found is important. Recording the location and position of the fingerprint supports later analysis.

The importance of fingerprint quality

Finger and palm prints are usually left at a crime scene by chance which can often result in a limited number of prints of varying quality. This frequently leads to partial marks, like the side or tip of a finger, being deposited. However, low-quality and partial prints can be used to establish someone’s identity as long as the recovered print contains enough information.

Analysing fingerprints – opinion not fact

Although fingerprint evidence was presented in court for many years as fact, the forensic regulator now classifies it as expert opinion. Even when experts work in the same organisation and use the same equipment, techniques, and approaches, they can make different judgements on the same evidence.

When fingerprints are analysed and compared, the outcomes can be classified into four groups:

1. Identified – the mark can be confidently attributed to a particular individual.
2. Excluded – enough fingerprint features do not match to conclude the marks did not come from the same person.
3. Inconclusive – the fingerprint mark does not contain enough detail for a viable comparison.
4. Insufficient – the fingerprint mark contains such low-quality detail that a reliable comparison cannot be made.

A specialist should be able to explain the limitations of fingerprint evidence to a legal team or jury.

Opportunities to challenge fingerprint evidence

When you receive a streamlined forensic report (SFR) issued by a fingerprint bureau, it will report initial fingerprint evidence findings. Different bureaus record information differently, and SFRs may not give the full picture.

Information you might expect to see includes:

• Which finger, thumb, or palm print at the crime scene has been identified as the crime scene mark.
• The origin of the crime scene mark, usually including a crime scene number.
• Details of other marks previously identified in the case.

You may also find additional SFRs if other marks are compared later. When fingerprint evidence is classified as inconclusive, legal teams can bring in a fingerprint expert to carry out their own analysis.

Context

Marks used to identify the presence of someone at a scene. For example, someone who lawfully visited the scene could have left a print in a location that eliminates them from the inquiry.

Length of time

The moisture content of a fingerprint can have a major impact on how long it lasts on a surface. Factors influencing persistence include composition, amount of matter deposited, surface properties, handling, position, and environmental conditions.

Currently, there is no reliable technique to accurately determine the age of a latent fingerprint.

What can influence fingerprint analysis outcomes?

A fingerprint identification is open to challenge and can occasionally be wrong. For instance, a fingerprint in a serious US terrorist incident was initially verified by three experts but later found to be incorrect. Misidentifications, though rare, do occur.

Probability models

Attempts to use statistical models to predict likelihoods of a match have failed, largely because they cannot account for the wealth of friction ridge detail that experts consider in their analysis.

How fingerprint experts ensure their opinion is valid

Independent analysis

For an expert’s opinion to stand in court, the analysis needs to be independent. Notes from previous experts should not be shared, to avoid jeopardising results. The expert will make their own notes and only view previous ones if there is a difference of opinion.

Objectivity

Fingerprint experts must remain objective. The Forensic Science Regulator recognises that cognitive biases can influence fingerprint decision-making. Experts mitigate bias by following guidance and undertaking training.

Making the most of fingerprint evidence

Working with a qualified and highly experienced fingerprint expert can add huge value to your legal team. Get in touch with our casework team on 01235 774870 or at science@forensic-access.co.uk.