Identifying & Collecting Physical Evidence

What is physical evidence? May be defined as any object, impression, or body element that can be used to prove or disprove facts relating to an offence. It is especially valuable because it often carries greater weight in court than evidence obtained through witnesses’ statements. Physical evidence is examined and analyzed by forensic scientists found at the scene. Forensic science is the application of biochemical and other scientific techniques to criminal investigation.

Impressions Impressions are patterns or marks found on various surfaces, caused by different objects such as gloves, fingers, shoes, tires, or tools. Impression collection is done in two stages: 1. It is recorded by photographing or scanning it, or taking a mould. 2. The police try to match the impression with the object that made it, such as matching a fingerprint to a suspect.

Characteristics of Impressions Class characteristics: general attribute of an object, such as type, model, style, and size. Lack specific details that might narrow down the range of possibilities to one particular item. Individual characteristics: specific and unique features of an object. These can narrow the range of possibilities considerably and help investigators make a positive id.

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; however, fingerprints are the easiest to work with and classify. Fingerprint patterns never change and are unique to the individual. No two people have ever been found to have the same fingerprint. Even identical twins have different fingerprints. Gloves do conceal criminal’s fingerprints but officers can use the glove impression to make an identification.

2 Types of Fingerprints A visible fingerprint – observable by the naked eye and is usually formed when the fingerprint is coated in dirt, grease, or other substance. A latent fingerprint – made by the perspiration and oils that naturally forms on the skin surface but cannot be seen by the naked eye. Print has to be developed before photographed. There are 3 methods to do this: Non-absorbent surfaces (metal or plastic) can be dusted by using a graphite power that sticks to the ridges. Lifted by using adhesive tape and placed on a white cardboard surface. Iodine fuming is used to life prints from absorbent surfaces such as paper and cloth. Existing fingerprints will absorb the iodine and become visible. Laser beams can illuminate the print. Sweat compounds on the surface will absorb the laser and the print turns yellow.

Shoe Prints and Tire Tracks Shoe and tire impressions can help place the suspect at the scene of the crime. Police can learn a great deal of information about the suspect if they can find 4 shoe imprints (2 from the left and 2 from the right). Can help determine height and weight, walking or running, and whether or not they were carrying anything.

Body Elements and DNA Crimes against people often result in the transfer of bodily fluids or other bodily elements from the suspect to the victim. These include blood, mucus, hair, and skin These bodily elements can be tested to match elements with a suspect. Blood is most commonly found at the scene. It is first tested to make sure it is human, then for blood type. Blood type will tell us if it is the victim or suspects but is not a positive identification without DNA testing. Fibers found at the scene can be matched to the suspects clothes, car, or home.

DNA DNA is the building block of a person’s genetic make-up. The DNA in everyone of our individual cells is identical and is unique to the ourselves with the exception of identical twins. Forensic scientists do not need a large sample to extract DNA. DNA samples can either link suspects with physical evidence or free them from suspicion.

Labeling Evidence Proper labeling ensures that evidence has not been contaminated or tampered with in any way. Chain of custody – is the witnessed, written record of all the people who had control over the items of evidence. It must show: Who had contact Date and time it was handled Circumstances under which it was handled What changes, if any, were made to the evidence Information that is included on the tag and evidence package – page 200