Identifying and Collecting Physical Evidence Section 8.2

Slide Title Physical Evidence – any object, impression, or body element that can be used to prove or disprove facts relating to an offence. Physical evidence is invaluable in court because it carries more weight with juries than witness statements. Forensic Science is the application of biochemical and other scientific techniques to criminal investigation. Although forensic scientists investigate crime scenes most of their work is done in a laboratory. They often are called into a trial to give expert testimony. Forensic scientists specialize in a number of fields from autopsies, firearms, chemists, blood splatter, etc.

Tools The most common tools used in crimes are hammers. Screwdrivers, and crowbars, which have individual characteristics on their surfaces or edges that can be detected in some laboratories.

Impressions Impressions are patterns or marks found on various surfaces caused by different objects such as fingerprints, gloves, shoes, tires, tools. The impression is photographed, scanned or by taking a mold. They then try to math it with the object that made it.

Impressions have two characteristics: Class Characteristics- the general attributes of an object. I.E. a tire may be 12 inches steel belted, belonging to a specific tire company. Individual Characteristics- the specific and unique features of an object. I.E. specific wear and tear on the left font tire caused by a bad wheel alignment.

Fingerprints A fingerprint is a patterned mark left on a surface by a fingertip. Prints can also be taken of a person’s hands, feet, or toes, but fingerprints are easier to work with and classify. Because fingerprint patterns never change and are unique to each individual, a fingerprint is the best type of impression to use to identify an offender. No two people have been ever found to have the same exact fingerprint. Even identical twins have different patterns. 

Two types of prints: 1) Visible fingerprint - can be observed by the naked eye 2) Latent fingerprint - cannot be seen; has to be developed These methods are used to develop latent prints: Dusted (with special powder); print lifted using adhesive tape Iodine fuming - used on paper and cloth to lift prints Laser beam - illuminates prints

Prints on non-absorbent surfaces, such as metal and plastic, can be dusted by using a graphite power that sticks to the ridges of the print. The print is then lifted using adhesive tape and placed on a white cardboard surface where it can be photographed. The officer who lifted the print initials the cardboard and it is stored for use later in court. The technique called ‘iodine fuming’ is used to lift prints from absorbent surfaces such as paper and cloth. The area being investigated is placed under iodine fumes; any existing fingerprints absorb the iodine and become visible. A laser beam can be used to illuminate the print. Sweat compounds deposited on the surface absorb the laser, and the print turns yellow and can be photographed.

Gloves Gloves do not necessarily hide a criminal’s fingerprints anymore. Police can use glove impressions to identify a suspect in almost the same way they use fingerprints. To make a positive identification, they compare the impression’s class characteristics with the gloves of a suspect.

Shoe Prints and tire Tracks Shoe and tire prints can be matched to a suspect and place them at the scene of a crime. If the police can find four foot prints, 2 tracks for each foot a lot of info can be uncovered. Such as: Height, weight, injuries, whether the offender was running or walking, if they were carrying anything, etc. Tire prints can be identified to prove make, model of vehicle, direction, etc.

Body Elements and DNA Crimes against people often result in the offender transferring bodily fluids (blood, semen, hair, skin) to the victim. Police use these substances in DNA testing - to match to a suspect. Blood is the most common substance found at a crime scene. It is valuable because it can eliminate and / or link suspects

DNA Evidence

DNA Cont….. Found in every cell in the human body - building block of person’s genetic make-up. Determines person’s physical characteristics (height, weight, hair colour). DNA is identical in every cell, but the patterns are different (except for identical twins) Scientists take samples and analyze them to create a profile to help identify a suspect. DNA Evidence has exonerated many Canadian of crimes they did not do, even after they had been convicted and sentenced to prison, in some cases, for many years.

Procedures for Labelling Evidence Officers must correctly label evidence so they may easily identified at a later date. Proper labelling ensures there was no tampering with the evidence. If it is not labeled properly, it could lead to the evidence being inadmissible in court. For this reason, a proper chain of custody must occur. The chain of custody must be unbroken and show: Who had contact with the evidence. The date/time the evidence was handled. The circumstances under which the evidence was handled What changes were made to the evidence.