TRACE EVIDENCE

LEARNING OBJECTIVES: Describe the methods and techniques used in finding and handling trace evidence. Explain how tool marks, serial numbers, laundry and dry-cleaning marks, bloodstains and body fluids, hairs and fibers, fingernail scrapings, and rocks and minerals should be handled as possible trace evidence. Describe the techniques used in the laboratory examination of trace evidence.

The techniques for finding and handling trace evidence during an investigation cover a large area. The following paragraphs do not coverall material items that could be classified as trace evidence, nor do they cover all investigative techniques that may be used for finding and handling such evidence. The points mentioned will alert you to other possibilities and, by association, suggest further development and perfection of investigative skills.

Trace evidence at a crime scene can include obvious items, such as bloodstains, or inconspicuous items, such as dust particles. All are easily overlooked, frequently mishandled, and all too often discarded as useless. Investigation files contain numerous case histories emphasizing the value of properly handling trace evidence in proving a case against a suspect or clearing an innocent person.

Be alert to the consequences of improper handling of trace evidence, particularly since such handling may completely negate the value of otherwise admissible court evidence. For example, if a suspect is returned to the scene of a crime before the scene is completely processed, the suspect could claim that the hairs found there were left during the return visit. The same assertion may diminish or negate the value of other trace materials found at the scene such as paint chips, fibers, dirt, and so forth.

Thoughtless intermingling of trace evidence found at different parts of the crime scene may also render valuable evidence worthless.

Always observe the cardinal rule for handling physical evidence, particularly trace evidence-avoid contamination.

Trace evidence may either be deposited at a crime scene by the perpetrator or may be carried away. The perpetrator may leave tool marks, bloodstains, hairs, fibers, soil, and similar traces, or may carry away bloodstains, hairs, fibers, glass fragments, soil, safe insulation, and similar traces on his or her person, clothing, or equipment.

You must keep these and other sources of trace evidence in mind and be diligent in your search for them at the scene, on the suspect, or in the area or on equipment used by the suspect. Unnecessary spectators should not be allowed at the crime scene as they will deposit material at the scene and/or destroy what trace evidence is present.

In rare instances, usually because of a lack of sufficient amount of material, the laboratory is unable to render any opinion concerning the evidence. Negative findings of this nature can be avoided if you follow the guidelines set forth concerning the amount of a specific item of evidence to collect.

TOOL MARKS

The credibility and acceptability of tool mark evidence by the courts has long been accepted.

"Courts are no longer skeptical that by the aid of scientific appliances, the identity of a person may be established by fingerprints. There is no difference in principle in the utilization . . . to determine that the tool that made an impression is the same instrument that made another impression. The edges of one blade differs as greatly as the lines of one human hand differs from lines of another...."

The above is an excerpt from the case of State v. Clark, 287 PAC 18 (1930).

Definitions

A tool mark is an impression, cut, scratch, gouge, or abrasion made when a tool is brought into contact with an object. A tool mark may be classified as a negative impression, as an abrasion or friction-type mark or as a combination of the two.

Negative impression is made when a tool is pressed against or into a receiving surface. This type of mark for example, is usually made when a crowbar is used to pry open a door or a window.

An abrasion or friction mark is made when a tool cuts into or slides across a surface. This type of mark may be made by a pair of pliers, a bolt cutter, knife, ax, saw, drill, plane, or a die used in the manufacture of wire.

A combination mark is made, for example, when a crowbar is forcefully inserted into the space between a door and the door facing and pressure is applied to the handle of the tool to force the door open. The forceful insertion of the crowbar produces an abrasion or friction mark and the levering action produces a negative impression.

Basis of Tool Mark Identification

Because no two tools are alike in every detail, they will not leave identical impressions. Tools may have obvious differences in size, width, thickness, or general shape. They also have minute differences that become apparent only when the tools are examined _{microscopically.} These minute differences may be caused by manufacturing, finishing, and grinding; by uneven wear; by unusual use or abuse; by accidents; by sharpening; or by alterations or modifications made by owners or users of the tools.

On the basis of these obvious and minute differences, it may be possible to identify the tool that made a given impression.

Uses of Tool Mark Evidence

Tool mark evidence may be used to do the following:

1. Link a person who uses a given tool with the crime scene, the commission of a crime, or some act material to a crime.

2. Establish whether a given tool or weapon found at a crime scene made a mark that is material to the crime. This knowledge is of value to you, whether or not the owner or possessor of the tool is known, because

it may eliminate the necessity of tracing a tool that, even though found at the crime scene, has no connection with the crime.

3. Establish a connection between similar evidence discovered in a series of crimes.

4. Determine whether a door or window was forced open from the inside or the outside.

5. Compare a tool mark from a crime scene with a tool mark found on the property, equipment, or vehicle of a suspect.

6. Facilitate and narrow the search for a given tool or weapon.