Section 1.2: Evidence Law & Investigations

Fundamentals of Criminal Investigation by Adam J. McKee

In the complex world of criminal investigations, evidence is the star of the show. It’s what investigators look for, what prosecutors present in court, and what juries use to decide if someone is guilty or not. But not all evidence is created equal. Let’s break down the different types and how they’re used to build a case.

Types of Evidence

First up, we have real evidence. This is anything you can touch, see, hear, smell, or taste—essentially, anything that appeals to your five senses. It’s the physical stuff that comes to mind when you think of evidence, like a piece of clothing, a weapon, or a bag of stolen money. But real evidence isn’t just objects. It also includes documents, photos, videos, sound recordings, and even computer files. Then there’s demonstrative evidence, like diagrams or simulations, created by investigators to help explain complex details of the crime to a jury.

Testimonial Evidence: The Power of Words

The second major player is testimonial evidence. This involves anything a person says under oath in court. It could be a witness talking about what they saw, an expert explaining technical details, or a defendant giving their side of the story. Outside the courtroom, statements like affidavits (written statements sworn to be true) and depositions (testimonies given under oath before the trial) also count, as long as they’re officially sworn. If it’s not under oath, it’s usually not allowed in court, though there are some exceptions to this rule.

Direct vs. Circumstantial Evidence

Evidence can also be categorized as direct or circumstantial. Direct evidence is like a straight line from point A to point B. If a witness says, “I saw him do it,” and they’re believable, that’s direct evidence. Video footage showing the crime as it happens is another example. It’s strong stuff because it doesn’t require the jury to make any leaps of logic.

Circumstantial evidence, on the other hand, is more like a connect-the-dots puzzle. It requires the jury to make inferences and draw conclusions. For instance, if someone’s fingerprints are found on a weapon, it suggests they handled it. This type of evidence requires a bit more thinking and piecing together, but it can be just as convincing, especially when there’s a lot of it pointing in the same direction.

Building a Case with Circumstantial Evidence

Even though direct evidence gets a lot of attention for its clear-cut nature, most cases rely heavily on circumstantial evidence. This can show everything from the defendant’s ability to commit the crime to their intent. Let’s look at some examples:

  • Ability and Intent: Circumstantial evidence can prove that a defendant had the means, knowledge, and opportunity to commit a crime. For example, finding a manual on bomb-making in a suspect’s home undermines the defense that they didn’t have the knowledge to build a bomb.
  • Modus Operandi (MO): This Latin term means “method of operation.” If a crime is committed in a way that matches the defendant’s known pattern, it suggests they’re responsible.
  • Motive: Understanding why someone might have committed a crime can help convince a jury of their guilt. Motives can range from financial gain to personal vendettas.
  • Threats: If someone threatened to commit a crime and then that crime happens, it can imply they followed through on their threat.
  • Acting Guilty: Actions like fleeing from police, trying to hide evidence, or being found with items stolen during a crime can all suggest guilt.

🔍 Reflect

How does the distinction between direct and circumstantial evidence affect your understanding of how criminal cases are built and decided?

Understanding the Rules of Evidence

In the courtroom, not everything that glitters is gold, especially when it comes to evidence. Just because something might shed light on a case doesn’t mean it will automatically be allowed in court. There are specific rules in place to guide what can and cannot be used during a trial. Let’s unpack these guidelines to see how they ensure fairness and justice in the legal process.

The Exclusionary Rule: Keeping It Legal

One of the cornerstone rules is the exclusionary rule. Imagine if someone snooped through your belongings without permission—that wouldn’t be fair, right? The exclusionary rule applies this principle to legal investigations. It says that evidence must be obtained legally to be used in court. This rule is rooted in the Fourth Amendment of the U.S. Constitution, which protects citizens from unreasonable searches and seizures. Basically, if the police want to use evidence they’ve found, they need to have followed the law to the letter when obtaining it. If there’s any doubt about whether a search was legal, the best bet is to have a warrant.

Fruits of the Poisonous Tree: The Ripple Effect

Closely related to the exclusionary rule is the “fruits of the poisonous tree” doctrine. This idea suggests that if the initial evidence was obtained illegally, then any further evidence found as a result of the initial evidence is also tainted and can’t be used in court. It’s like a bad seed growing into a tree that only bears bad fruit. This doctrine underscores how one illegal action can compromise an entire case, potentially letting the guilty party walk free.

The Admissibility Trio: Relevant, Material, and Competent

Even if evidence is obtained legally, there are three more hurdles it must clear to be considered admissible in court: relevancy, materiality, and competency.

  • Relevancy: Evidence must directly relate to the case at hand. It needs to prove or disprove something important to the case. If it doesn’t, it might distract or mislead the jury, so it’s left out.
  • Materiality: This is about the significance of the evidence. It must be substantial enough to impact the case’s outcome—like a key piece of a puzzle that helps make the picture clear.
  • Competency: This refers to the reliability of the evidence. If there’s reason to doubt the evidence or its source, it might not be used. The goal is to maintain the trial’s integrity by ensuring only trustworthy evidence is considered.

🔍 Reflect

How do you think the rules of evidence affect the fairness and outcome of trials?

The Importance of Chain of Custody

Imagine evidence as a relay race baton that needs to be passed carefully from one runner to the next. The chain of custody in criminal investigations is somewhat similar. It’s all about keeping a meticulous record of who holds the evidence, from the moment it’s collected until it’s presented in court. This process is crucial to ensure the evidence remains untampered and credible.

What Is Chain of Custody?

Chain of custody is a legal safeguard designed to maintain the identity and integrity of evidence. It creates a documented timeline of the evidence, noting every person who handled it, when it was handled, where it was collected, and a detailed description of the item. This meticulous documentation ensures that the evidence presented in court is the same as what was initially collected, without any alteration or tampering.

How Is Chain of Custody Maintained?

Here’s how the custody of evidence is typically safeguarded:

  • Physical Possession: The evidence is considered secure when it is directly in someone’s physical control.
  • Secured View: Once it leaves direct physical control, it must remain in a secured view or locked away, ensuring it remains unchanged.
  • Restricted Access: Keeping the evidence in a secured area where only authorized personnel can access it is crucial.
  • Minimal Handling: Limiting the number of people who collect and handle the evidence reduces the risk of mishandling or tampering.
  • Documentation: Every time the evidence changes hands, the transfer is documented. This includes who received the evidence, when, and under what circumstances.

Why Is Chain of Custody Critical?

The integrity of the evidence is paramount in criminal justice. If there’s any break in the chain of custody—if the evidence could have been tampered with or altered in any way—it might not be admissible in court. That’s because the credibility of the evidence is everything; if it’s compromised, the entire case could be jeopardized. Ensuring a seamless chain of custody verifies that the evidence presented to the court is as original and untouched as possible.

Maintaining the chain of custody isn’t just about following protocols; it’s about ensuring justice is served correctly. It’s a fundamental process that underpins the reliability of evidence and, by extension, the fairness of the trial itself.

🔍 Reflect

Considering the crucial role of chain of custody in ensuring evidence integrity, how do you think its mishandling could impact the outcome of a criminal case?

Understanding Linkage Concepts

Linkage concepts in criminal investigations are like the threads that connect different pieces of a puzzle, helping to form a clear picture of what happened at a crime scene. These concepts revolve around comparison samples—pieces of evidence that, when matched with others, can link suspects, tools, and locations to a crime. Let’s dive into how these concepts help in solving crimes.

Comparison Samples: The Basics

Comparison samples are essentially physical evidence collected from crime scenes, suspects, tools, or locations that can be compared to establish connections. There are two main types of comparison samples:

  1. Unknown/Questioned Samples: These are samples found at a crime scene without a clear source. They might be left by anyone involved in the crime—be it the suspect or the victim. The goal is to match these unknown samples with known ones to establish a link. Examples include fingerprints found at a crime scene or DNA from a hair or bloodstain.
  2. Known Samples: These are samples with a documented source, used to compare against the unknown/questioned evidence to establish a connection. Known samples are divided into:
    • Standard/Reference Samples: These are verified samples taken for comparison purposes, such as fibers from a suspect’s carpet or paint chips from a vehicle.
    • Control/Blank Samples: These are uncontaminated samples related to the crime scene but not involved in the crime. They’re used to ensure that the testing environment doesn’t interfere with the results, like an unstained segment of carpet from the crime scene.
    • Elimination Samples: These come from individuals who had lawful access to the crime scene, helping to rule out innocent sources of evidence, like fingerprints from the home’s residents or tire marks from police vehicles at the scene.

Linking Crimes, Suspects, and Scenes

The power of comparison samples lies in their ability to link crimes to suspects or to connect different crime scenes to the same perpetrator. For instance, if fibers from a victim’s clothing match fibers from a suspect’s home, it suggests a connection between the suspect and the victim. Similarly, matching tire treads found at different crime scenes can indicate the same vehicle was present at each location, pointing to a single perpetrator behind multiple crimes.

The Role of Linkage Concepts in Investigations

Linkage concepts serve as a foundational strategy in criminal investigations, enabling detectives to piece together evidence from various sources to form a coherent narrative of the crime. By systematically comparing known and unknown samples, investigators can identify suspects, corroborate testimonies, and sometimes link seemingly unrelated cases.

These concepts underscore the importance of meticulous evidence collection and analysis, ensuring that every piece of evidence, no matter how small or seemingly insignificant, is considered. This approach not only aids in solving individual cases but also helps in identifying patterns of criminal behavior, ultimately aiding in the prevention of future crimes.

🔍 Reflect

How do you think the process of comparing known and unknown samples contributes to the integrity and effectiveness of criminal investigations?

Key Terms

References and Further Reading


Modification History

File Created:  05/02/2019

Last Modified:  04/03/2024

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