Forensic Handbook 1 – Marks at the Crime Scene

Dr Allison Jones

May 2002

Thanks to books, films and TV, many people may consider themselves to be forensically aware these days. Fibres, hairs, skin cells, body fluids and fingerprints are well-known sources of forensic evidence used to convict criminals. But there are other, lesser-known, forensic sources often present at crime scenes. Indeed, criminals often leave a variety of marks in the wake of their wrongdoings. And footwear marks and tool marks are two common examples.

Case Study

Leah and Milton Rosenthal, a wealthy couple who owned homes in Paris and Israel, disappeared in 1981. Their killer was convicted partly on the strength of multi-stroke tool mark evidence.

Leah was last seen at her 27-year-old son Danny's home in London. Danny, a reclusive paranoid schizophrenic, was suspected by police to be involved in his parent's disappearance. A search of his home revealed certain 'unhealthy interests', which included conducting surgical experiments on live chickens. However, amidst all the bird blood in his 'laboratory', forensic scientists found traces of human blood. They also found a hacksaw with traces of human blood and fragments of bone in the locking nut. The blade was missing.

Meanwhile, police in France discovered the dismembered body of a male matching the description of Milton, not far from his Paris apartment. A search of the apartment revealed more traces of human blood, microscopic bone fragments – and a hacksaw blade.

In collaboration, a French pathologist and an English forensic scientist compared saw marks on the victim's bones to the teeth of the hacksaw blade. They matched.

London police traced the hacksaw and identified it as having been bought by Danny Rosenthal a few weeks earlier. The evidence was enough to convict him of murder. The body of Mrs Rosenthal has never been found, but police suspect she met the same fate as her husband.

Tool marks

Tool marks may be found at a range of crime scenes, including house burglaries, car thefts and firearms offences. Criminals often need to force entry when committing crimes. Many different tools are used for this task, all usually leaving behind some indication of their use. Usually in the form of a distinguishing mark.

How tool marks are caused

Tool marks are caused by the interaction of two objects. The harder object will leave marks on the softer object. The nature of these marks is determined by: the type of tool used; the relative hardness of the two surfaces; the force applied; the motion of one surface over the other. Tool mark types are put into four categories.

'Impressed' (or 'static marks') are caused when a harder object is pressed into a softer object. For example, a screwdriver used to open a window will be pressed into the softer window frame and will leave an impressed mark.

'Striated' (or 'dynamic marks') are caused by two objects sliding across each other. The harder object leaves scratch marks on the softer object. A key scraped along a car, for example.

'Crush marks' result from pressure being applied on opposite sides of an object. This causes a ridge of compressed material. Made by tools like wire cutters.

'Multi-stroke marks' are caused by the repetitive action of a tool, like a saw or a file.

Characteristics of tools

Forensic scientists consider tools to have three characteristics.

'Class characteristics' are the features that give the tool its size and shape. They are shared by all tools of a particular type, and are inherited at the manufacturing stage.

The class characteristics of a screwdriver

'Sub-class characteristics' will be shared by some tools of a particular type but not all of them. Generally, these features are accidentally introduced during the manufacturing stage.

The sub-class characteristics of a screwdriver

'Individual characteristics' may be caused by isolated incidents during manufacture but will certainly be caused later on during use. They are unintentional and will result from wear and tear.

Recording the information

Tool mark evidence includes the mark left at the scene and also the tool itself – often recovered from the suspect or found discarded at the scene. Where possible, the object with the tool mark is taken to the laboratory. Where this isn't possible, the mark is photographed and then cast using silicone rubber.

When tools are collected, great care is taken not to damage them in transit. Any damage to the tool would alter the characteristics and could render comparisons inconclusive or even impossible. Tools are packed individually and secured so they can't move.

Reading the signs

The initial examination of a tool and tool mark is to identify the class characteristics. If a tool mark's class characteristics match a suspect tool, the mark will be examined further for sub-class and individual characteristics. The tool and tool mark are examined side-by-side, using a comparison microscope.

The concept of matching individual characteristics is based on the theory that those characteristics occur randomly on tools. The probability of two tools having exactly the same characteristics is considered to be insignificantly small. As with footwear, the pattern of marks on tools is regarded as unique.

Footwear marks

Footwear marks might not seem like an obvious source of evidence. Particularly as they can be difficult to see and may be present along with footwear marks unrelated to the crime. But they can be unique and, if discovered, can provide evidence that a particular shoe was present at the scene of the crime.

Footwear marks can also provide much information about the crime itself. They may be able to reveal:

• the number of people at the scene
• the direction of approach and departure to and from the scene
• the points of entry and exit to and from any premises
• the mode of entry to any premises
• the sequence in which the events took place

How marks are left behind

Three phenomena occur when someone walks across a surface: static charges are created; the surface deforms; there is an exchange of materials between the shoe and the surface.

In soft surfaces, such as mud or snow, a permanent three-dimensional impression is formed. But on hard surfaces, such as carpet or tiles, any impression is temporary, as the surface will quickly return to its original shape. However, there will often be a two-dimensional residue of marks on hard surfaces due to the transfer of trace materials. Static charges picked up by footwear facilitate this transfer.

Footwear marks are hence either two or three-dimensional depending on the surface.

Finding the marks

Approach and departure, entry and exit, and the scene itself are all taken into account when searching for footwear marks. Entry and exit point marks may be found on window sills, surfaces inside windows, kicked-in doors, broken glass and flower beds.

However, footwear marks are often not found for several reasons. This may be because the surfaces aren't good for producing footwear marks or because the marks are obliterated, either by the offender, sometimes intentionally, or by weather conditions.

Recording the information

When a footwear mark is found, distance photographs will first be taken to show the position of the mark relative to its surroundings. The mark is then photographed alongside a rule before any attempt is made to cast or lift it. The scale of the rule is later used so the photograph can be enlarged to the actual shoe size.

Where possible, objects with footwear marks on them are removed and taken to the laboratory. But where this isn't possible, an impression of the mark is taken at the scene.

Three-dimensional casting is an important part of footwear examination. A cast can reveal more than a photograph, as minor details that may not show up in a photograph have more chance of being captured in a cast.

Two-dimensional marks are recovered by lifting them onto another surface, like metallic film. This is to give a greater contrast between the mark and its background, enabling more detail to be seen. Lifting is done by Electrostatic Lifting Apparatus (ESLA), gelatine lifters or adhesive lifters.

Reading the signs

When studying footwear marks the focus is on sole pattern, wear and tear signs, and random damage marks.

There is much variation in the patterns of soles, even within a particular footwear brand. By consulting a database, the sole pattern may lead detectives to the make and model of the shoe. If the marks are partial, an estimate of the shoe size may be made.

The marks may show individual characteristics of the footwear in question. These characteristics occur at random and no other shoe will have exactly the same ones. They are wear and tear signs such as worn down areas and scuffs, and random damage marks such as cuts and 'inclusions' (eg small stones and pieces of glass).

Therefore, just as a fingerprint is unique to an individual, a footwear mark is unique to a shoe. If marks found at a crime scene match marks of a particular shoe, it's evidence that the shoe was at the scene.

Find out more

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Websites

Crime Team
Channel 4's Crime Team lifts the lid on grisly murders from the crime history books.

Bite marks
www.crimelibrary.com/forensics/bitemarks/
The story of how bite mark testimony has been used to convict criminals such as serial killer Ted Bundy.

Tread marks
www.k9forensics.com/treadmark%20story.htm
Commercial website describing a new system of footwear analysis and identification. Includes a brief history.

Forensic Handbook 2 – Trace Evidence
A forensic theory says that every contact leaves a trace. A burglar might leave hair behind and take carpet fibre with them. But he might take more besides. Like paint and glass. Find out how this can help detectives solve crimes.

Forensic Detectives and Crime Fiction
The forensically-minded sleuth is a popular character these days. But in fact the crime genre has a had long history of operating on the cusp between art and science. A 150-year-old history. Read about it here.

Books

	Fingerprints by Colin Beavan Account of the history of fingerprinting, culminating in the 1905 trial when two brothers were convicted on the evidence of a thumbprint. Get this book
	The Casebook of Forensic Detection: How science solved 100 of the world's most baffling crimes by C Evans A beginner's guide to history of forensics. Features 100 of the most absorbing and ground-breaking cases in forensic history. Get this book
	The Real Cracker by Stephen Cook Explores the work of real-life offender profilers as they investigate five horrific recent crimes. How do they compare to the public's image inspired by Fitz, TV's larger-than-life psychologist? Get this book
	Mammoth Book of Murder and Science by Roger Wilkes Forensic science has provided the key to some of this century's most complex and intriguing cases. This book examines cases where traditional methods of detection have failed. Looks at scientific routes of investigation and analysis that have led to the conviction of many dangerous criminals. Get this book

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