Criminal Investigation: The Evidence
Chain of Custody & Evidence
The collection of evidence begins a process called the chain of custody. The chain of custody is a written, witnessed record that describes:
Chain of Custody: A written record of everyone who collected, handled, and studied evidence from a crime scene. This "paper trail" proves that the evidence collected from the crime scene is the same evidence that is submitted later in court.
Establishes proof that the evidence that is collected is the same evidence that is presented later in a court of law. If it is not maintained, the defense might argue that somebody could have tampered with the evidence. The judge will then generally rule that this evidence cannot be used.
Circumstantial evidence: Suggests that someone is linked to the crime, but it doesn't prove he or she did it.
Think back to the crime described at the beginning of this module:
In that crime a bullet removed from the victim's body matched ones from his wife's gun. While this may suggest the wife shot him, it does not prove it. The evidence is circumstantial because someone else may have fired the gun.
Direct evidence: Is proof or a fact to do with the crime.
For example, if someone saw the wife shoot her husband, or if she was caught by the police in the act, that would be direct evidence.
In court, more weight is obviously given to direct evidence. However, if enough circumstantial evidence can be gathered that points to one suspect, then it's possible the judge or jury can be convinced that the suspect is guilty.
- Who handled the evidence.
- When each person handled it (date and time).
- Why the evidence was handled.
- What changes, if any, were made to the evidence from the time it was collected until the time it is presented in court.
Chain of Custody: A written record of everyone who collected, handled, and studied evidence from a crime scene. This "paper trail" proves that the evidence collected from the crime scene is the same evidence that is submitted later in court.
Establishes proof that the evidence that is collected is the same evidence that is presented later in a court of law. If it is not maintained, the defense might argue that somebody could have tampered with the evidence. The judge will then generally rule that this evidence cannot be used.
Circumstantial evidence: Suggests that someone is linked to the crime, but it doesn't prove he or she did it.
Think back to the crime described at the beginning of this module:
In that crime a bullet removed from the victim's body matched ones from his wife's gun. While this may suggest the wife shot him, it does not prove it. The evidence is circumstantial because someone else may have fired the gun.
Direct evidence: Is proof or a fact to do with the crime.
For example, if someone saw the wife shoot her husband, or if she was caught by the police in the act, that would be direct evidence.
In court, more weight is obviously given to direct evidence. However, if enough circumstantial evidence can be gathered that points to one suspect, then it's possible the judge or jury can be convinced that the suspect is guilty.
Educational Videos Introducing Crime Scene Investigations
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Searching for Evidence
Collecting 3-Dimensional Impressions
When investigating a crime scene, an identification officer may find impressions such as footprints, tire tracks, or tool marks.
Impression:
An indentation or imprint left on a surface by a person, object, or vehicle.
Impressions can reveal important clues about a crime; therefore, they need to be collected very carefully.
To collect three-dimensional impressions (e.g., footprints in snow or tire tracks in mud), the IO will make a cast.
To make a cast, he will pour a liquid into the impression then wait for it to harden.
When it's removed, the hardened cast provides a copy of the impression.
Impression:
An indentation or imprint left on a surface by a person, object, or vehicle.
Impressions can reveal important clues about a crime; therefore, they need to be collected very carefully.
To collect three-dimensional impressions (e.g., footprints in snow or tire tracks in mud), the IO will make a cast.
To make a cast, he will pour a liquid into the impression then wait for it to harden.
When it's removed, the hardened cast provides a copy of the impression.
Cast:
A physical copy of an impression created by pouring a liquid into the impression then waiting for it to harden.
Dental Stone:
Sprays a fixative such as shellac or lacquer onto the impression then waits for it to harden.
In snow or ice: will use snow print wax as a fixative instead; this is to prevent the impression from melting.
Next, pours dental stone into the impression, waits for it to set, then removes it.
Dental stone does not shrink, so it creates an exact copy of the original impression.
Silicone:
If the impression has small cracks or crevices, the IO might cast it with silicone.
Silicone begins as a paste, but then hardens.
When it's solid, silicone is still flexible enough that it can be easily pulled out without breaking.
Foam:
Foam impressions are used most often when trying to match a suspect's footprints with ones found at the crime scene.
IO will have the suspect step into a box of soft foam, leaving behind a shoe impression.
IO then compares this to the collected print to see if they match.
A physical copy of an impression created by pouring a liquid into the impression then waiting for it to harden.
Dental Stone:
Sprays a fixative such as shellac or lacquer onto the impression then waits for it to harden.
In snow or ice: will use snow print wax as a fixative instead; this is to prevent the impression from melting.
Next, pours dental stone into the impression, waits for it to set, then removes it.
Dental stone does not shrink, so it creates an exact copy of the original impression.
Silicone:
If the impression has small cracks or crevices, the IO might cast it with silicone.
Silicone begins as a paste, but then hardens.
When it's solid, silicone is still flexible enough that it can be easily pulled out without breaking.
Foam:
Foam impressions are used most often when trying to match a suspect's footprints with ones found at the crime scene.
IO will have the suspect step into a box of soft foam, leaving behind a shoe impression.
IO then compares this to the collected print to see if they match.
Collecting 2-Dimensional Impressions
Collecting a two-dimensional impression (e.g., a dusty shoeprint on a hard floor) is very similar to fingerprinting.
IO applies a revealing agent or development medium to the impression then lifts it with special tape or a machine.
Tape:
After applying the revealing agent the IO may use transparent cellophane tape to lift the print.
This method is particularly good on curved surfaces.
Gelatin Lifters:
A gelatin lifter has a backing with sticky gel on it.
IO places the sticky gel over the impression, smoothes it, then lifts a copy of the impression.
He then places a clear cover over the impression.
Lifting Materials:
Although the impression may not be visible to the naked eye, when it is taken to the lab and analyzed under special lighting, the details emerge.
Gelatin lifters work well on solid, dry surfaces such as wood floors.
Electrostatic Lifter:
An electrostatic lifter is a foil sheet with a black back connected to a device that generates a large amount of static electricity.
Electrostatic lifters work well on non-porous floors such as wood or linoleum.
IO shines a light to locate the print.
He then places the foil sheet over the surface and applies an electrical charge to it.
This causes dust from the print to attach to the foil.
IO then removes the foil sheet, and places a transparent cover over the print to preserve it.
IO probably will not know what type or brand of shoe made the impression.
To find out, he compares his unknown sample against known samples (shoes he can identify) until he locates one that has the same class characteristics.
Once he finds a match he knows what type and brand of shoe left the impression at the crime scene.
IO applies a revealing agent or development medium to the impression then lifts it with special tape or a machine.
Tape:
After applying the revealing agent the IO may use transparent cellophane tape to lift the print.
This method is particularly good on curved surfaces.
Gelatin Lifters:
A gelatin lifter has a backing with sticky gel on it.
IO places the sticky gel over the impression, smoothes it, then lifts a copy of the impression.
He then places a clear cover over the impression.
Lifting Materials:
Although the impression may not be visible to the naked eye, when it is taken to the lab and analyzed under special lighting, the details emerge.
Gelatin lifters work well on solid, dry surfaces such as wood floors.
Electrostatic Lifter:
An electrostatic lifter is a foil sheet with a black back connected to a device that generates a large amount of static electricity.
Electrostatic lifters work well on non-porous floors such as wood or linoleum.
IO shines a light to locate the print.
He then places the foil sheet over the surface and applies an electrical charge to it.
This causes dust from the print to attach to the foil.
IO then removes the foil sheet, and places a transparent cover over the print to preserve it.
IO probably will not know what type or brand of shoe made the impression.
To find out, he compares his unknown sample against known samples (shoes he can identify) until he locates one that has the same class characteristics.
Once he finds a match he knows what type and brand of shoe left the impression at the crime scene.
Characteristics of the Evidence
Class Characteristics: Physical features that are common to a group of similar objects.
Looking for a Match
Lots of people will have that same type and brand of shoe in their closet.
Any one of dozens or hundreds or perhaps thousands of people potentially could have left that shoe print at the crime scene.
No two person’s shoes, even if they are the same type and brand, will leave the exact same impression.
–Every person’s shoes has a different wear pattern based on how the person walks, the surfaces the person walked on, how old the shoe is, and so on.
All of these things make the soles of your shoe take on what are called accidental characteristics.
SFIS then looks for a specific pair of shoes from the known class that has the same accidental characteristics as the unknown object
–e.g. same nicks and tears on the outsole as on the unknown pair in the footprint
If they finds a match they will conclude that someone wearing the known shoes created the impressions found at the crime scene.
Accidental Characteristics:
Cuts, tears, and other marks that distinguish an object from others in its class.
Looking for a Match
Lots of people will have that same type and brand of shoe in their closet.
Any one of dozens or hundreds or perhaps thousands of people potentially could have left that shoe print at the crime scene.
No two person’s shoes, even if they are the same type and brand, will leave the exact same impression.
–Every person’s shoes has a different wear pattern based on how the person walks, the surfaces the person walked on, how old the shoe is, and so on.
All of these things make the soles of your shoe take on what are called accidental characteristics.
SFIS then looks for a specific pair of shoes from the known class that has the same accidental characteristics as the unknown object
–e.g. same nicks and tears on the outsole as on the unknown pair in the footprint
If they finds a match they will conclude that someone wearing the known shoes created the impressions found at the crime scene.
Accidental Characteristics:
Cuts, tears, and other marks that distinguish an object from others in its class.
Important Terminology
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