Crime Scene Investigation: Evidence, Photography, and Legal Principles

Crime Scene - Basic Principles

1. Preserve life/health/safety

2. Scene security (no unauthorized persons)

3. Establish boundaries/limit access

4. Search for evidence

Types of Evidence

Mostly physical + testimonial evidence

value comes from content + context

Trace Evidence

Small amounts of material that transfer (e.g., fibers, hair)

Latent Evidence

Evidence not visible without enhancement

Patent Evidence

Visible evidence (e.g., blood prints)

Crime Scene Priorities

1. Life/safety

2. Secure scene from unauthorized access

Crime Scene Boundaries

Use large boundaries to ensure all evidence is included

Focal Length

Determines angle of view (longer focal length = narrower view)

Aperture

Size of lens opening that controls amount of light

Shutter Speed

How long shutter stays open

slower = more light but more blur

Film Speed (ISO)

Sensitivity to light

higher speed = less light needed but grainier

Depth of Field

How much of the scene appears in focus

Overall Photos

Wide-angle photos of the entire scene

Midrange Photos

Photos showing specific areas of interest

Close-up Photos

Photos of individual pieces of evidence

Photography Tradeoffs

Wide aperture = shallow depth

slow shutter = motion blur

high ISO = grainy images

Lossy Compression

Compression that loses detail (e.g., JPEG)

not preferred for crime scenes

White Balance

Adjusting camera so white appears white under different lighting temps

Parallax Error

Image distortion from rendering 3D → 2D

affects size/shape accuracy

Oblique Lighting

Side lighting used to highlight texture and 3D features

Crime Scene Sketch Purpose

Shows relative positions of items

contemporaneous note

Crime Scene Sketch Elements

Label, scale, distances, legend, cardinal direction

Chain of Custody

Written record of every person who handles evidence

Identification (Forensics)

Determining origin with strong certainty

Comparison (Forensics)

Comparing pattern features for similarity/difference

Class Characteristics

Group-level features

Individual Characteristics

Unique features identifying one source

Fourth Amendment

Protects against unreasonable search/seizure

requires warrants

Warrants

Generally required unless in public space or exception applies

Exclusionary Rule

Illegally obtained evidence may be excluded from trial

Crime Scene Reconstruction

Using evidence to infer events

Inference

Logical conclusion based on evidence

Inductive Reasoning

Specific → general (small to big)

Deductive Reasoning

General theory → specific predictions

Problem of Induction

Cannot logically prove an induced theory

future data may contradict

Karl Popper - Falsification

A theory can be disproven by one adverse observation

must be falsifiable to be scientific

Confirmation Bias

Favoring evidence that supports preconceived beliefs

Bifurcation

Oversimplifying complex issues into yes/no choices

Overgeneralization

Assuming what's usually true must be true in a specific case

False Linkage

Assuming items are related without evidence

Area of Convergence

Droplet tails point in direction of travel

lines from multiple droplets locate origin

Droplet Shape - Direction

Vertical impact = round

shallow angle = elongated

Angle of Impact

Width ÷ Length = sin(angle)

Low Velocity Spatter

Large, well-defined drops

High Velocity Spatter

Mist-like droplets (e.g., gunshot)

Cast-Off Spatter

Blood flung from a swinging weapon

often an arc pattern

Back Spatter

Blood from entry wound moving back toward shooter

Arterial Spurt

Pulsing, high-pressure spurts synchronized with heartbeat

Cause of Death

Medical explanation of biological process of death

Manner of Death

Legal determination of circumstances of death

Homicide

Death caused by deliberate action of another

Suicide

Death caused intentionally by self

Accident

No intent

accidental cause

Natural

Disease or medical process

Undetermined

Manner cannot be determined

Rigor Mortis

Muscle stiffening after death

Livor Mortis

Discoloration from pooling blood

Algor Mortis

Cooling of the body after death

Autolysis

Self-digestion of cells after respiration stops

Putrefaction

Microbial decomposition of tissues

Common Postmortem Samples - Femoral Vein

Easy to locate

peripheral blood less subject to postmortem change

Common Postmortem Samples - Ocular Fluid

Closed system

stable after death

Forensic Entomology

Using insects to estimate postmortem interval (PMI)

Blowflies

Typically first insects to colonize remains

lifecycle stages give PMI estimate

Maggot Masses

Indicate perimortem wounds or openings

Arthropod Artifacts

Some insects leave marks mistaken for injuries

Forensic Anthropology

Analyzing bones for sex, height, and trauma timing (antemortem/perimortem/postmortem)

Anatomical Sex Estimation

Pelvis + skull most important

Height Estimation

Possible from long bones

Anthropometry

Measurement of anatomical features for ID

Friction Ridges

Ridges on hands/feet creating impressions

Fingerprint Class Characteristics

Loops, whorls, arches

Fingerprint Minutiae

Lakes, islands, spurs, bifurcations, pores

ACE-V

Analyze, Compare, Evaluate, Verify

Toolmarks

Hard object creating marks on softer object

plastic deformation/scoring or material transfer

Bullet

Projectile fired from gun

Cartridge Case

Metal case holding bullet, propellant, primer