Crime Scene Investigation and Fingerprinting Methods

Every person in the world has a unique set of ridges on their fingers. These are formed in the third and fourth month of pregnancy and remain unchanged for life - apart from size. In his book, Invisible Evidence, Bill O'Brien discusses the characteristics of fingerprints and how these are exposed at a crime scene.

The Characteristics of Fingerprints

Fingerprints are friction ridges that enable people to grasp things. They appear in three basic patterns:

• · Loops
• · Whorls
• · Arches

There are no sebaceous glands in the skin of the hand but there is a greater concentration of sweat glands than in other parts of the body. When a person sweats, the mixture is 99 percent water and one percent fats and acids. The more a person sweats, the better the fingerprints left behind. Once the water has evaporated, the fats and acids remain in the form of a fingerprint.

How Fingerprints are Revealed at a Crime Scene

For over 100 years, fingerprints have been exposed by carefully brushing a fine powder across the area. Black powder is used on light-colored surfaces and white powder is used on dark surfaces. The print is then lifted with tape and placed on a clear acetate sheet.

In a laboratory setting, a fuming tank may be used to expose fingerprints on non-absorbent and semi-absorbent items such as bottles, cans, plastic, glossy cardboard and tinfoil. It works by heating commercial-grade superglue to 100°C. These fumes are circulated through the tank and stick to the fingerprint, so setting it and making it firm. It is then treated with dyes to make it clearly visible.

Absorbent surfaces such as wood, paper and plain cardboard tend to suck in fingerprints meaning a different technique must be used to expose them. Latent print examiners use ninhydrin, a chemical that helps detect amino acids that are found in sweat. After being mixed with a solvent, ninhydrin is applied to the surface in question and the result is bright purple fingerprints.

Other chemicals and powders are used to reveal fingerprints on sticky surfaces such as adhesive tape and on thermal paper such as is used for till receipts and ATM slips.

What is the Automated Fingerprint Identification System

The Automated Fingerprint Identification System is commonly referred to as AFIS and is often mentioned in television programs such as CSI. In simple terms, it is a computer system that stores fingerprints of criminals and compares these to fingerprints found at a crime scene.

It takes the computer a few minutes to locate possible matches depending how many points of reference are available. These matches will then be checked manually by a fingerprint expert.

Fingerprints from crime scenes can be stored on AFIS even if no match is found - and in some cases the culprit reoffends and is caught and his fingerprints are matched to previous crimes.

Fingerprinting is a basic tool in crime investigation and is effective in proving that a person was present at a crime scene. Using a variety of methods, investigators can expose fingerprints on virtually any surface and these are recorded in AFIS to aid in future crime scene investigations.

Reference:

O'Brien, Bill, Invisible Evidence, David Bateman Ltd, 2007