Semester Exam Review Criminal investigations

crime scene

Any location where a crime has occurred or is suspected to have occurred

what is the primary goal of crime scene processing?

to identify, document, preserve, and collect physical evidence in its original, pristine condition to help reconstruct the events, establish facts, link suspects/victims to the scene, and ultimately support the investigation and prosecution of a crime.

What is the role of the first responder at a crime?

stop any potential threats, provide first aid to those who need it, establish a perimeter and identify witnesses.

Why is contamination dangerous to an investigation?

it compromises evidence which leads to false results, wrongful convictions, or dismissed cases, wasting time and resources by introducing irrelevant material

Why must the scene be photographed before evidence is moved?

to ensure accurate documentation that aids investigation, attorneys juries in understanding the sequence of events

What items should be documented before evidence is collected?

photographs, videos and detailed notes

Why should victims and witnesses be separated at a crime scene?

Victims and witnesses must be separated at a crime scene primarily to prevent contamination of their independent accounts of the event and to prevent witnesses to work together to create a false story.

How does weather affect crime scene processing?

Weather significantly impacts crime scene processing by degrading or destroying evidence like DNA, footprints, and blood spatter, forcing investigators to secure the scene quicker using protective measures or sometimes waiting for better conditions.

What risks occur if too many officers enter a scene?

Excessive personnel movement at a crime scene increases the risk of cross-contamination of valuable forensic evidence, such as DNA traces, which can compromise the investigation and future prosecution efforts and too many officers can lead to confusion regarding roles, responsibilities, and chains of command.

What is the difference between chain of command and chain of custody?

Chain of command is an organizational hierarchy that defines authority and reporting lines, while chain of custody is a documented, chronological process for tracking physical evidence or valuable assets to maintain their integrity. 

Why is the chain of custody essential for court?

The chain of custody is essential for court because it proves evidence hasn't been swapped, tampered with, or contaminated, ensuring its reliability and authenticity from collection to courtroom.

What happens if the chain of custody is broken?

If the chain of custody is broken, evidence can become inadmissible, potentially leading to dismissed charges.

Why is the chain of custody especially important for DNA?

The chain of custody is crucial for DNA evidence to prove its authenticity, and reliability, ensuring it hasn't been contaminated, swapped, or tampered with from collection to court presentation, which prevents legal challenges and keeps results admissible.

Can evidence still be used if the chain is broken? Explain.

No, if the chain of custody is broken, evidence is often ruled inadmissible in court because its authenticity and integrity are compromised, making it unreliable to prove guilt or innocence.

Why do investigations rely on multiple types of evidence?

Investigations rely on multiple evidence types to build a reliable, comprehensive picture of events

How can one piece of evidence fit into more than one category?

One piece of evidence can fit into multiple categories because the categorization framework is often determined by the purpose for which the evidence is being used or the specific criteria of the system being employed.

Which types of evidence are most likely challenged in court?

Testimonial evidence (witness credibility), digital evidence (due to alternation risks), and even forensic, while generally strong challenges can arise over collection and contamination.

Why is physical evidence stronger than testimonial evidence?

Physical evidence is more reliable because its a real object and when collected and analyzed properly, it offers concrete connections to a crime scene or person. while testimonial evidence can be influenced by memory, perception and external factors. It´s easily questioned on the stand through cross examination regarding credibility and memory.

Which type of evidence degrades the fastest?

Transient evidence degrades faster because it is temporary and expected to disappear quickly.

Testimonial evidence

Testimonial evidence is a statement made by a witness under oath to establish the truth of a matter in question, and it is a type of evidence presented verbally in court or in a deposition

Direct evidence

Direct evidence is proof that directly establishes a fact without requiring any inference or presumption, such as an eyewitness account or a video of the event. It is a form of proof that directly proves a point in a case

indirect ( Circumstantial) evidence

Indirect evidence, also known as circumstantial evidence, is proof of a fact from which another fact can be reasonably inferred, rather than directly proving it.

Physical evidence

Physical evidence is any tangible item or substance that links an offender to a crime or proves a fact, like hair, fingerprints, DNA, blood, fibers, footprints, weapons.

Transient evidence

Transient evidence in forensics refers to physical evidence that is temporary and easily lost, altered, or degraded by time or environmental conditions such as temperature and odor.

Class evidence

Class evidence is forensic evidence that links an object or person to a general group rather than a single source such as blood type.

Individual evidence

Individual evidence is any evidence that can be linked to a single, specific source, such as a particular person or object.

How does stress affect eyewitness accuracy?

stress significantly degrades the quality and completeness of eyewitness memory, making accounts less reliable for identification and detail recall. 

Why might two witnesses describe the same event differently?

Two witnesses describe the same event differently because of different vantage points or memory reconstruction. Leading each person to perceive, interpret, and recall different details, even filling in gaps with assumptions or prior experiences. 

Why are confessions considered testimonial evidence?

Confessions are considered testimonial evidence because they are out-of-court statements made by the suspect about their own involvement in a crime, functioning as a direct admission of guilt.

Can DNA be circumstantial? why?

Yes, DNA evidence is often considered circumstantial because it links a person to a place or object but doesn't directly prove they committed the crime.

Which type of evidence requires inference?

The type of evidence that requires inference is circumstantial evidence, also known as indirect evidence, which doesn't directly prove a fact but relies on logical deductions and connecting other established facts to reach a conclusion.

Why do most cases rely heavily on circumstantial evidence?

Most cases rely on circumstantial evidence because criminals often leave no direct proof, so prosecutors build a strong case by piecing together indirect clues

How does improper packaging affect testing?

Improper packaging of physical evidence compromises testing by causing contamination, deterioration, loss and damage.

Why must biological evidence be preserved differently than weapons?

Biological evidence degrades easily due to environmental factors and contamination, requiring strict control like air-drying, paper packaging, and climate-controlled storage.

How can physical evidence contradict testimony?

Physical evidence contradicts testimony by providing objective facts (like DNA, fingerprints, video, or forensic analysis) that clash with a witness's subjective account, revealing inconsistencies in details (e.g., timing, location, cause).

Why is contamination a major threat?

Contamination is a major threat to physical evidence because it compromises integrity, leading to unreliable results, wrongful convictions.

Why is transient evidence considered time-sensitive?

Transient evidence is considered time-sensitive because it is temporary and can easily change, degrade, or disappear over a limited period due to environmental conditions, natural processes, or the actions of people and the environment. 

Why can transient evidence never be retested?

Transient evidence can never be retested because, it is temporary and will naturally degrade, change, or disappear over time if not immediately and properly preserved.

How does blood drying help investigators?

Blood drying helps investigators by creating Bloodstain Pattern Analysis clues, revealing a timeline, identifying the type of injury linking suspects , and reconstructing events through stain characteristics like shape, size, and the skeletonization of edges as they dry.

Why must transient evidence be photographed immediately?

Transient evidence must be photographed immediately because it's temporary and easily lost, altered, or destroyed by environmental factors (rain, wind, heat), so photos capture its original state, location, and context before it vanishes, preserving crucial details like footprints, odors, or body temperature that link suspects to a crime. 

How does weather affect transient evidence?

Weather significantly impacts transient evidence (like odors, blood, footprints, fibers) by degrading, altering, or destroying it.

Why is DNA considered individual evidence?

DNA is considered individual evidence because, except for identical twins, every person has a unique genetic blueprint, making it a powerful identifier that can definitively link a suspect to a crime scene.

Why is blood type always class evidence?

Blood type is class evidence because it categorizes people into large groups rather than uniquely identifying one person.

Why can serial numbers turn class evidence into individual evidence?

Serial numbers can turn class evidence into individual evidence because they provide a unique, traceable identifier that is specific to a single object

Which carries more weight in court and why?

In court, the "weight" of evidence depends on its persuasiveness and credibility, with trustworthy sources, and direct evidence often carrying more weight than biased or unverified information.

Why is class evidence still valuable?

Class evidence remains valuable because it narrows down suspects, provides crucial context, statistically increases the probability of a suspect's presence , and helps eliminate innocent individuals.

What can BPA reveal about a crime?

Bloodstain Pattern Analysis reveals the story of a violent crime by examining the size, shape, and distribution of bloodstains to reconstruct events, showing the location of victims/assailants, type/force of weapon, number of blows, sequence of events, and if a cleanup occurred.

Why is directionality important?

Directionality is crucial in Bloodstain Pattern Analysis because it reveals the flight path of blood droplets, helping to determine the Angle of Impact and the Area of Origin (where the bloodshed started), providing vital clues about the sequence of events, location of the victim/suspect, type of weapon used, and actions.

Why is surface texture important?

Surface texture is crucial because it dramatically changes a blood drop's final size, shape, and edge characteristics, affecting interpretation.

why is BPA both science and interpretation?

Bloodstain Pattern Analysis (BPA) is both science and interpretation because it relies on physics, biology, and math (science) to understand how blood behaves, but requires trained analysts to interpret those patterns to reconstruct events.

How can BPA disprove a suspect´s statement?

Bloodstain Pattern Analysis (BPA) disproves a suspect's statement by scientifically reconstructing the events of a crime, using bloodstain size, shape, location, and distribution to reveal inconsistencies with a suspect's story, such as proving a victim was stationary (refuting self-defense) or showing the perpetrator's movement and position, thereby creating a physically impossible scenario for the suspect's account.

Passive Bloodstain

A passive bloodstain pattern is created by the force of gravity acting on a blood source, resulting in stains like single drops, flows, and pools. These patterns form without external force and provide information about a victim's movement or the source of the blood. 

Transfer Pattern

A transfer bloodstain pattern is created when a bloodied object comes into contact with a surface that has no blood on it, transferring blood from one to the other. These patterns can include a mirror image of the object, such as a bloody fingerprint or shoe print, or a swipe/wipe pattern.

Cast-off

A cast-off blood pattern in forensic science is a trail of blood drops flung from a moving, blood-covered object, like a weapon or limb, creating linear streaks on surfaces, revealing the object's swing path, direction, and even type of weapon.

Impact spatter

Impact bloodstain patterns are created by an external force, such as a weapon, hitting a blood source, which projects blood through the air. These patterns include high, medium, and low-force spatter, and their analysis can help forensic investigators determine the type of force, the direction of travel, the number of blows, and the location of the incident by examining factors like the size, shape, and direction of the bloodstains.  

Arterial spray

large quantity of blood spurting due to a severed artery.

Expirated blood

Expirated bloodstains are blood forced out of the nose, mouth, or a wound by airflow, creating distinctive patterns often mixed with saliva or mucus, showing tiny air bubbles (bubble rings/beading), appearing diluted/lighter, and traveling farther/differently than impact spatter.

Bloodstain Pattern Analysis

The study of bloodstains at a crime scene to reconstruct events.

What type of pattern forms by gravity?

Which reveals repeated blows?

The bloodstain pattern that reveals repeated blows is a cast-off pattern. 

Which contains air bubbles?

Expirated bloodstain pattern

Which indicates a severed artery?

Arterial spray

Which links suspects directly to services?

Transfer pattern

What category indicates violence?

Which forms without force?

Passive Bloodstains

Which shows surface contact?

Transfer Stains

Why must stains be correctly categorized?

Proper categorization allows analysts to form conclusions about what happened, verify witness statements, determine the type of weapon used, and identify the positions and movements of individuals involved. 

Why can blood type exclude but not identify a suspect?

Blood type can exclude a suspect because a person cannot have a blood type they didn't genetically inherit; however, it cannot definitively identify them because many people share the same blood type. 

What blood type is the universal donor?

The universal donor blood group for red blood cells is O negative

Why is contamination dangerous in blood testing?

Contamination in blood testing at a crime scene is dangerous because it mixes foreign DNA with the real sample, creating mixed profiles, false positives, or masking the true DNA, which leads to misleading results, wrongful convictions, or wasted investigations, making evidence unreliable and jeopardizing justice.

Which pattern is most common?

The most common fingerprint pattern is the Loop

Which pattern has no deltas?

The Arch fingerprint pattern has no deltas

Which pattern has two or more deltas?

The Whorl has two or more deltas?

Why are loops classified as ulnar or radial?

Loops are classified as radial or ulnar based on the direction their ridges flow, named after the forearm bones

Radial Loop

a radial loop opens towards the thumb (radius bone side)

Ulnar Loop

an ulnar loop opens towards the little finger (ulna bone side)

Plain Whorl

The simplest whorl, featuring a circular or spiral pattern where an imaginary line between the two deltas crosses at least one of the inner ridges.

Central Pocket Loop Whorl

Similar to a plain whorl but with a tighter central pocket; the imaginary line between deltas does not cross any inner circulating ridges.

Double Loop Whorl

Contains two separate loops that intertwine, often looking like an "S" or yin-yang symbol, with two distinct cores and two deltas

Accidental Whorl

A complex pattern that doesn't fit into the other categories, often combining two or more different patterns (excluding a plain arch) or having multiple deltas. 

What is a minutiae?

the tiny, unique ridge characteristics (like ridge endings, splits, dots) in fingerprints that forensic examiners use to identify or exclude a person

Why is minutiae more important than pattern type?

because minutiae are highly unique, random details formed during development, creating distinct points for matching

Why don´t twins share identical minutiae?

Identical twins don't share identical fingerprint minutiae because these tiny ridges form due to a mix of genetics (general pattern) and random, unique environmental factors in the womb.

How many matching minutiae are usually required?

8-16 are generally considered sufficient for a fingerprint match in court

Why is fingerprint classification necessary?

Fingerprint classification is necessary because it organizes the huge number of fingerprints in databases (criminal, civil) into manageable groups, allowing for fast and efficient searching to link suspects to crimes, verify identities, and build criminal histories.

Why is it used before AFIS?

Fingerprint classification was used before AFIS (Automated Fingerprint Identification Systems) for manual organization, sorting prints into broad categories to narrow down massive physical card files, making searches feasible but slow.

Why does classification save investigative time?

Fingerprint classification saves investigative time by dividing massive databases into smaller, manageable groups (like loops, whorls, arches).

Why does it prevent errors?

Fingerprint classification prevents errors by reducing a massive database to a manageable subset, making matching faster and more efficient, and by creating a structured system.

How does it help repeat offenders be identified?

Fingerprint classification helps identify repeat offenders by creating a unique code for each person's fingerprints allowing law enforcement to systematically file and quickly search vast databases.

Why was Henry classification created?

to organize vast numbers of fingerprint records for efficient criminal identification.

What problem did Henry´s classification solve?

Henry's Classification System solved the problem of organizing massive fingerprint records

Why is a universal system necessary?

A universal fingerprint system is necessary for seamless, rapid, and comprehensive identification across different jurisdictions and agencies, preventing criminals from hiding by using different names, enhancing national security, solving cold cases with latent prints, and ensuring faster background checks for employment.

What does AFIS stand for?

Automated Fingerprint Identification System

Why does AFIS not make final identification?

AFIS doesn't make final identifications because it's a search tool, not a judge; it provides a candidate list of potential matches (often 10 or more) based on complex algorithms and features (minutiae), but human examiners must manually compare these prints, confirm the quality, and make the final decision.

Why do states maintain their own databases?

States maintain their own databases primarily due to the principles of federalism, operational control, and data sensitivity. 

How do state and federal systems work together?

State and federal fingerprint systems work together through a linked network, primarily via the FBI's Interstate Identification Index (III), where states send fingerprints ( to the FBI's Next Generation Identification system, creating a national repository for positive identification across jurisdictions, enabling searches for criminal history, background checks, and linking crimes by sharing data through established agreements, all built on shared, fingerprint-verified records. 

Why are fingerprints still used alongside DNA?

Fingerprints are still used with DNA because they often appear more frequently at crime scenes (on surfaces like glass, metal) and provide immediate links to suspects in databases

Why does the Fourth Amendment protect citzens?

The Fourth Amendment protects citizens by ensuring their right to privacy and security against unreasonable government intrusion.

What is probable cause?

Probable cause is a legal standard in the United States that requires law enforcement to have a reasonable belief that a crime has been committed or that evidence of a crime exists in a specific location.

Why are warrants required in most cases?

Warrants are required in most cases to protect citizens from unreasonable government intrusion, stemming from the Fourth Amendment, ensuring searches and seizures have judicial oversight based on probable cause.