Incriminating Marks

- r' - 4 &

Tools feature in many different crimes. marks They can be used to force open a door or Incriminating window, break a padlock, or even to cot up a corpse. By cutting, striking, and witnesses. No No No DNA. A skilled, fingerprints. footprints. scraping, tools can leave marks on every surface they touch. These marks, if clear Nosmart, and methodical criminal knows just what investigators are enough, have identifying features that can looking for, and works hard to stop them from finding it. However, even be matched to the tools that made them. the used and the worn can leave subtle tools gloves Types of tool marks There are two basic kinds of tool marks- marks behind. Investigating these marks can multiple and single contact marks. help police link a suspect with a crime scene Multiple contact marks occur when a surface is repeatedly sawed. They have limited value as evidence, and may only demonstrate what class of tool was used, such as a saw or knife, and its general size and shape. Single contact marks occur 4 when a surface is struck once. They can be either impressions, such as when a hammer dents a metallic surface, or striations, such as when a screwdriver scrapes against a window frame to prize it open. Striation marks are parallel indentations left on a surface, as shown in the above right-hand image. Their unique pattern can provide proof that a particular tool produced a mark. However, the tool's owner may not have committed the crime, I since tools may be stolen, borrowed, or - .- I simply found at the crime scene. Making their mark r S - Tool marks may show tiny surface imperfections of a tool's working edge. These imperfections are created both during manufacturing and through f7_# general wear and tear while the tool is in use. defects 1 Manufacturing may appear on every tool of the same batch, so although they can help to trace a tool's source, they cannot necessarily provide investigators with a positive match. By contrast, wear on a tool can make it unique-impressions in soft materials faithfully reproduce some 1) of the tiniest chips, dents, and - scratches. Though this is

4. BREAKING THE CHAIN %./ Here, a chain link has been cut using bolt cutters. By y the cut at a F' : examining ( microscopic level, It marks from the .01 ' tool can be seen.

'-4 \ RN::. SW Ør INCRIMINATING MARKS

Examination and comparison a chain, only a small portion of the blade Examiners compare tool-mark evidence actually cuts the metal. Using trial and with corresponding implements recovered error it would be difficult to find the from a suspect. An initial examination correct portion of the blade to match with and measurement using a low-power marks on the chain. However, chemical stereo microscope (see p. 89) is usually spot tests can reveal traces of the chain's r enough to eliminate tools that clearly metal, and pinpoint a region of the blade could not have made the mark. But if for comparison. Other traces left on tools there are conspicuous similarities, these can help to prove guilt. Pliers used to cut can be confirmed by using the tool to telephone cable, for instance, may retain P. duplicate the marks found at the crime traces of the plastic insulation. scene. For example, examiners might cut a lead or aluminum rod with the bolt cutters they BOLT CUTTER MATCH A suspect a criminal used to open Microscopic ridges, known as strioe, ore left on o cut a a soft chain link at the scene (A). The suspect's bolt cutters, padlock. (By cutting pressed into soft lead, match these marks exactly (B). metal, they reduce the risk _airiiiTV of marking the tool itself, but since damage is always one of the principal advantages of tool possible, this is the last test marks, it can also be a limitation. If a examiners carry out.) The tool is in regular use, its value as evidence more points of similarity / falls with each day that passes after the there are on the two marks, crime is committed. Eventually, new the more compelling the marks obliterate old ones, so many evidence that the same tool crime bureaus delete tool mark evidence made both.

from their databases after six months. Investigators rarely t If imperfections in a tool are very study tool marks in isolation. pronounced, they can leave cleat traces Instead, they analyze the even on multiple contact marks. For marks in conjunction with example, a saw with a broken tooth will other trace evidence from leave a characteristic gap when it cots. the scene that may have transferred to the tool. This LAB ANALYSIS A and tool marks to the marks and often Finding recording helps interpret Bolt cutters recovered from o suspect ore analyzed in By their very nature, tool marks occur results in a more convincing case. For the lob. As well as looking forstriation marks, where force has been used. Investigators example, when bolt cutters shear through technicians examine the tool for any trace evidence. look for them at a crime scene's point of entry-a forced window or door, or a cot FABRIC PRINTS padlock on a perimeter fence. They also Marks from fabrics can be collected and Glove left on The on this find them wherever damage has been done prints point in ways similar to o windowpane ore finger leaves a or a tool was obviously necessary-such compared fingerprints recovered using distinct pattern break a and footprints-by dusting, tape lifts, and as on the limbs of dismembered corpse, in the electrostatic lifts Marks are fingerprint powder. glove's print, or in the slash in a vandalized car tire. (see p. 20). more distinctive if the material is coarse, The ideal method of collecting and are made from The tool-mark evidence is to remove the mark commonly gloves. prints of brand-new gloves at a crime scene itself from the crime scene-for example provide few clues. However, like tools, by removing a forced door, or a portion gloves accumulate unique features with of it, for later examination. If this is not use. In this image, a glove is compared to a practical, investigators photograph the mirror-image of a print. The rips and dried 4V - 1 mark, lighting it from an oblique angle paint on the glove's fingers help match it to to the surface detail. may highlight They the distinctive print. Fabric prints also occur also an resin, which cast it using opaque in hit-and-run road accidents. The textured retains detail much better microscopic weave of a victim's clothing can leave a than the or dental stone plaster usually patterned imprint on the vehicle's hood. used when casting footprints (see p. 20).