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Starting a Police Investigation
What happens when the police first arrive at the scene of a crime? What roles do different officers play? What preliminary steps do they have to take to carry out their investigation? These questions will be answered in the following sections.
Arriving at the Crime Scene The location or site where an offence takes place is referred to as the crime scene. When officers arrive, they have three tasks to perform. Their first task is to call an ambulance and assist injured people at the scene. They must also call in reinforcements to help eliminate any hazards that still pose a risk, such as fires or unexploded bombs. Finally, officers must continue to search the crime scene even if witnesses say the perpetrators have left. To protect people at the crime scene, the officers must assume that the perpetrators are present and armed; once police have thoroughly searched the scene, they can assume it is safe.
Protecting and Preserving the Crime Scene
The Crown's success in prosecuting offenders often depends on the condition of the physical evidence taken from the scene of the crime. To protect the crime scene, the officers must accurately establish two boundaries: the centre and the perimeter. The centre of the crime scene is the area in which the offence was actually committed. The perimeter consists of the surrounding areas where the offender may have been present or may have left evidence. These areas include any entry or escape routes used by the offender. Crime scenes are preserved for three reasons: to allow for a thorough search of the scene, to seize and collect physical evidence, and to ensure that the physical evidence seized is admissible in court. If evidence obtained at the crime scene is not managed properly, it can become contaminated. Contamination is the loss, destruction, or alteration of physical evidence. Contaminated evidence may not be admissible in court, and it may lead the police to draw inaccurate conclusions. One way to protect and preserve evidence is to document the scene carefully and accurately. Investigators keep a police log, which is a written record of what each officer has witnessed at a crime scene or has learned from questioning witnesses or suspects. Officers use their logs to document their daily activities. Later, these logs will help officers recall events, particularly when they testify at a trial. Investigators also use photographs, sketches, and other recording techniques to document the evidence found at a crime scene.
Officers' Roles at a Crime Scene
Four types of police officers investigate a crime scene, and each officer has a separate and well-defined role to play. For example, a "patrol officer" has a "beat" or an area that he or she checks regularly. Usually the patrol officer is the first member of the police department to arrive at a crime scene. The officer's primary duty is to secure the crime scene and ensure that no evidence is lost or tampered with. The officer will usually wrap yellow police tape around the perimeters of the crime scene and conduct the initial inter- views with witnesses to the crime. Patrol officers may also arrest suspects if they come upon a crime in progress.
A "scenes of crime officer" is trained in evidence collection and preservation techniques. These officers are usually skilled photographers, trained in lifting fingerprints and foot and tire impressions. They also collect blood and hair evidence. Scenes of crime officers tend to work on less serious offences such as break and enters and car thefts. A “criminal identification officer" is responsible for searching the crime scene, examining the scene for physical evidence, gathering and analyzing evidence, and sending certain types of evidence to a laboratory for analysis.
For instance, if a murder victim has struggled with the murderer, a criminal identification
officer may collect bits of the murderer's skin from under the fingernails of the victim and
send them to a lab to be analyzed.
A "criminal investigations bureau officer" is a plainclothes detective with experience in
a particular area of crime, such as homicide, robbery, or sexual offences. These officers
are trained to supervise the investigation, interview witnesses, interrogate suspects, draw
conclusions from the physical evidence, and arrest suspects.
Building Your Understanding
- What three tasks must the police complete when they first arrive at a crime scene?
- What two boundaries must the police establish at a crime scene? How do these
boundaries help their investigation?
- Provide an example of a piece of contaminated evidence and explain why it would
have an adverse impact on the Crown's case against an accused.
- Briefly explain why the police keep a log of their activities at a crime scene.
- Compare the roles played by a patrol officer and a criminal identification officer.
IDENTIFYING AND COLLECTING PHYSICAL EVIDENCE
In many criminal trials, the Crown must be able to prove beyond a reasonable doubt that the accused was present at the crime scene when the offence was committed. For this reason, the collection, preservation, and analysis of physical evidence is a crucial aspect of police work. 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. This type of evidence is especially valuable because it often carries greater weight in court than evidence obtained through witnesses’ statements. Forensic science is the application of biochemical and other scientific techniques to criminal investigation. Forensic scientists examine and analyze the physical evidence found at a crime scene. They do most of their work in laboratories, but they also spend considerable time giving expert testimony at trials and inquests. Perhaps the best-known type of forensic scientist is the medical doctor who performs autopsies to determine a murder victim's cause and time of death. Some scientists specialize in firearms and are able to analyze bullet fragments or gunshot residue to identify the type of gun used in a crime. Forensic chemists can also determine the type of vehicle driven by the offender by examining a paint chip left at a crime scene, and entomologists (insect specialists) can
- 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 Criminals who use gloves to conceal their fingerprints may be in for an unpleasant surprise. 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 (such as the overall pattern of the glove and the spacing of its stitches) and the individual characteristics (such as worn or torn, areas) with the gloves of a suspect.
Shoe Prints and Tire Tracks Shoe prints and tire tracks can be matched to le suspect's shoes or tires to help place the suspect at the scene of the crime. If the police can find four shoe prints-two of the left foot and two of the right-they can learn an amazing amount about a suspect. These prints can help them determine the suspect's approximate height and weight, any injuries he or she might have sustained in committing the crime, whether the suspect was carrying anything, and whether he or she was walking or running. Tire impressions can help investigators determine the type of tires, the make of car, and the direction in which the car was travelling as it entered or left the crime scene.
Fast Fact Fingerprint patterns never change and are usually the last surface to decompose after death.
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. Such elements include blood, semen, mucus, sputum, hair, and skin. The police can use any of these substances far DNA testing and other laboratory tests in order to match the elements with a particular suspect. Blood is the most common body substance found at a crime scene. Investigators send blood samples for laboratory analysis to determine whether the sample is, in fact, human blood. If it is human blood, then the sample is further analyzed to see whether it matches the blood type of the victim or suspect. Because blood types are like class characteristics, they cannot be used far purposes of positive identification without DNA testing. Hair and clothing fibers can be easily transferred from an offender to a victim during the commission of a crime. The police can use these fibres to match those found in a suspect's clothes, car, or home. Investigators can then take representative fibres from the suspect's belongings and submit them to a forensic laboratory for comparison.
DNA Testing What is DNA? DNA (deoxyribonucleic acid) is the building block of a person's genetic make-up. It is found in every cell in the human body and determines a person's physical characteristics, such as height, weight, and hair colour. The DNA of every cell in a person's body is identical. However, the pattern of the DNA is different for each person,
with the exception of identical twins. A forensic scientist can extract a person's DNA from as small a sample as ' a few drops of blood or a couple of hair fibres. This sample is then analyzed, creating a DNA profile that can be used in much the same way that fingerprints are used to identify a person. A suspect's DNA "print" can be compared with the DNA profile of a sample from the crime scene. If the profiles match, investigators will conclude that the two samples came from the same person. If the profiles do not match, then the samples must have come from different people, and the investigators will have to find another suspect. Therefore, DNA analysis can be used to either link suspects with physical evidence or free them from suspicion.
Procedures for Labelling Evidence One primary task of scenes of crime officers is to label items of evidence so they can be easily identified at a later date. Proper labelling also ensures that the evidence has not been contaminated or tampered with in any way. For these reasons, the officers take pains to establish a proper chain of custody for the evidence. A chain of custody is the witnessed, written record of all of the people who had control over the items of evidence. This chain remains unbroken from the time the evidence is discovered at the crime scene to the time it is produced in court. It must show
-. who had contact with the evidence; -. the date and time the evidence was handled; -. the circumstances under which the evidence was handled; and -. what changes, if any, were made to the evidence.
All evidence collected at the scene of the crime is tagged and placed in an "evidence package." The following information usually appears on both the tag and the exterior of the package:
-. brief description of the item -. police case number -. date when the evidence was collected -. location of collection -. brand name of the item, if any -. serial number or clothing information -. name and badge number of the officer who collected the evidence -. destination of the item for analysis or storage
Fast Fact One important advantage to DNA testing is that DNA molecules are stable. If properly preserved, they can be used in an investigation decades after the crime occurred.
Consider This An error in labelling does not necessarily exclude the evidence. If the defendant's rights have not been violated, the judge can include the evidence on the basis that a "reasonable person" would not be shocked by any misconduct in evidence handling. Discuss whether judges should have this discretion.
