A Simplified Guide To Crime Scene Investigation
Introduction
Crime scene investigators systematically examine areas to uncover physical evidence.
The goal is to identify what happened and who was involved.
Careful and thorough investigation is crucial to collect evidence and avoid destroying fragile evidence.
Case investigators and crime scene personnel collaborate to:
Define and secure potential evidence areas.
Examine and document the scene.
Collect physical evidence.
Preserve, package, and submit evidence for lab analysis.
Investigators reconstruct crime elements using key evidence.
Thoroughness increases the likelihood of accurately determining case facts.
Evidence quality and handling impact attorneys' arguments and jurors' conclusions.
Principles of Crime Scene Investigation
Locard's Exchange Principle: Every contact leaves a trace.
Whenever someone enters or exits an environment, something physical is added to and removed from the scene.
Investigators link suspects to victims, objects, and scenes using this principle.
Associative Evidence: Links a person to a crime scene
Examples: fingerprints, blood, bodily fluids, weapons, hair, fibers.
Answers the question: "Who did this?"
Reconstructive Evidence: Helps investigators understand the actions at the scene.
Examples: broken window, blood spatter, bullet paths, shoe prints.
Answers the question: "How did it happen?"
Control Samples: Known substances collected to establish linkages.
Examples: fibers, glass fragments, soil, vegetation.
Circumstantial evidence if found on a suspect linking them to the scene.
Eliminating potential perpetrators is also important
Gauging the Value of Evidence
Uniqueness: Evidence that narrows down suspects or the manner of the crime is useful.
Example: Tire tread impressions to identify the type of tire.
Example: Shoe print tread to identify shoe size and type, and wear patterns to match a specific pair.
Low Probability of Chance Occurrence: Mathematical probabilities help determine the likelihood of coincidence.
DNA evidence matching a suspect has an extremely low probability of occurring by chance.
Even common evidence like a shoeprint can be valuable when combined with other evidence.
Inconsistency: Items out of place or inconsistent with the setting/victim can be important.
Example: A cigarette butt at the scene when the victim is a non-smoker.
Physical Match: Trace evidence on a suspect matching something at the scene is valuable.
Example: Broken plastic parts or a fingernail that can be matched by fracture marks.
Why and When a Crime Scene Investigation is Conducted
Physical evidence helps investigators recreate the crime scene and establish the sequence of events.
It can corroborate statements from victims, suspects, and witnesses.
Scenes investigated include:
Burglaries
Violent crimes
Suicides
Fires
Auto thefts
Auto accidents
The type of case dictates the evidence sought, but this can change as facts emerge.
Investigators may discover multiple crimes or scenes.
Primary Crime Scene: The original location of the crime.
Secondary Crime Scene: A location related to the crime but not where it initially occurred (e.g., body found in a field away from the actual death location).
How It’s Done
A wide variety of physical evidence can be collected that is valuable (“probative”) for collection and investigation:
biological evidence (e.g., blood, body fluids, hair and other tissues)
latent print evidence (e.g., fingerprints, palm prints, foot prints)
footwear and tire track evidence
trace evidence (e.g., fibers, soil, vegetation, glass fragments)
digital evidence (e.g., cell phone records, Internet logs, email messages)
tool and tool mark evidence
drug evidence
firearm evidence
For a burglary, the order is typically:
Photograph and document the scene
Collect trace materials (especially from points of entry)
Collect low-level DNA evidence by swabbing areas of likely contact
Collect other items that may contain biological evidence
Locate and collect latent fingerprints
Who Examines Crime Scenes
The number and type of professional(s) depend on the crime and agency resources.
Larger agencies have dedicated specialists; smaller agencies may use first responders or detectives.
A detective investigates, interviews, and pieces together information.
Crime scene personnel search and collect evidence.
The crime scene investigation team may consist of crime scene photographers and evidence collection personnel specializing in gathering specific evidence such as latent prints, DNA, trace evidence and the like.
In the United States, there are no national requirements that must be met to serve as a crime scene investigator
Investigators can achieve four levels of certification through the International Association for Identification (IAI) to demonstrate their proficiency:
Certified Crime Scene Investigator
Certified Crime Scene Analyst
Certified Crime Scene Reconstructionist
Certified Senior Crime Scene Analyst
Other certifications include:
Evidence Photographer Certification from the Evidence Photographers International Council, Inc.
Board Certified Medicolegal Death Investigator of the American Board of Medicolegal Death Investigators (ABMDI).
How a Crime Scene Investigation is Conducted
Circumstances dictate the approach, but these seven steps are often followed:
Establish the scene dimensions and identify potential safety and health hazards
Locate the "focal point" (main area of disturbance).
Establish a sizeable area to contain all relevant evidence.
Identify potential perpetrator entry/exit paths.
Address safety: weapons, biohazards, chemical hazards, traps.
Advise medical, fire, and coroners about evidentiary issues.
Establish security
Secure the area to control access using tape, cones, etc
Establish a common entryway for personnel.
Document everyone entering/leaving the scene.
Establish consultation and evidence storage areas if needed.
Plan, communicate, and coordinate
Develop a theory of the offense.
Anticipate potential evidence
Gather information from witnesses/persons of interest.
Develop an evidence-collection strategy considering weather, time, etc.
Request additional forensic resources as needed.
Conduct a primary survey/walkthrough
Prioritize evidence collection.
Identify valuable evidence.
Take notes and initial photographs.
Document scene conditions (lights, shades, furniture, smells, temperature).
Create an evidence-free pathway to the primary area of interest.
Document and process the scene
Conduct a thorough, coordinated investigation to collect all probative evidence.
Use detailed documentation with digital and video cameras or a 3-D scanner.
Create sketches and diagrams if needed.
Follow proper procedures for collecting, packaging, and preserving evidence, especially biological evidence.
Conduct a secondary survey/review
Conduct a second survey of the area as a quality control step to ensure that the scene has been thoroughly searched
Record and preserve evidence
Create an inventory log to account for all evidence.
Descriptions must match photos and the crime scene report.
Establish a chain of custody.
How and Where Tests on the Evidence are Conducted
Evidence is sent to a forensic lab or outside analyst.
Crime scene personnel may conduct initial screening tests, called presumptive tests, at the scene to identify valuable evidence.
Presumptive tests can determine the type of substance present (toxin, drug, body fluids) but are not specific enough to confirm specific compounds.
Presumptive tests allow investigators to narrow the field of possibilities to a certain class of substance, but they are not specific enough to confirm the presence of specific compounds.
Presumptive tests help:
Provide clues about the crime.
Reduce the quantity of evidence submitted to the lab.
Expedite processing.
As technology advances, more testing will likely be conducted at the scene.
FAQs
What kind of results can be expected from the crime scene investigation process?
Detailed documentation of the scene.
Collection of evidentiary items for analysis.
Improved forensic technologies increase the value of trace and biological evidence, especially DNA.
DNA profiling can link individuals to a crime scene with even small amounts of biological evidence.
The criminal investigation process takes time, with the initial investigation just the beginning.
The case investigator also relies on other types of evidence including eyewitness testimony to piece together the full picture of the crime.
What are the limitations of the crime scene investigation process?
Crime scenes can be chaotic, unlike the portrayal in media.
Weather can damage evidence.
Evidence requires forensic analysis for conclusive facts.
DNA or fingerprints may not identify a perpetrator if there are no suspects or database matches.
How is quality control and quality assurance performed?
Each step ensures a thorough, high-quality investigation.
A debriefing is conducted to ensure the investigation of the area is complete.
Chain of custody ensures evidence handling was proper.
A seamless chronological record must be created indicating each person who takes possession of a piece of evidence, the duration of custody and the security of the storage conditions.
Forensic labs have policies and procedures for facilities, equipment, methods, analyst qualifications, and training.
Accreditation verifies that a crime laboratory meets quality standards.
What information does the report include and how are the results interpreted?
A crime scene report is a supplement to the initial report completed by the investigating officer.
It includes:
Date/time when technicians arrive
Weather conditions at the scene
Perpetrator’s point of entry and exit (if this can be determined)
Theory about perpetrator’s movements/actions
List of evidence collected
List of photos/videos taken
Vehicle descriptions
Emergency medical personnel documents
Sketches/diagrams of the scene
List of related subjects (suspects, victim, others involved)
Are there any misconceptions or anything else about crime scene investigation that would be important to the non-scientist?
Crime scene personnel usually don’t also work in the forensic laboratory as well.
Analysts routinely specialize in one particular area of examination.
The majority of crime scenes investigated are not of a high-profile nature, like a homicide case.
Investigators spend the majority of their time collecting evidence from scenes of burglaries, robberies or lesser crimes.
Crime scene investigation is definitely not a glamorous activity, unlike how it is often portrayed in popular culture.
New tools are continually being introduced to allow crime scene personnel to more thoroughly examine, discover and recover evidence from a scene.
Crime scene investigation relies primarily on the skills and knowledge of the investigators and forensic scientists involved.
Common Terms
Alternate light source: Special lighting device that produces visible and invisible light at various wavelengths to help investigators locate and visually enhance items of evidence.
Associative evidence: Any evidence that can link a person or an item to the scene of the crime.
Biological evidence: Physical evidence such as bodily fluids that originated from a human, plant or animal.
Chain of custody: The process used to maintain and document the chronological history of the evidence.
Contamination: The unwanted transfer of material from another source to a piece of physical evidence.
Control sample: Material of a known source that presumably was uncontaminated during the commission of the crime.
Cross-contamination: The unwanted transfer of material between two or more sources of physical evidence.
Elimination sample: Material of a known source taken from a person who had lawful access to the scene.
First responders: The initial responding law enforcement officer(s) and/or other public safety official(s) or service provider(s) arriving at the scene prior to the arrival of the investigator(s) in charge.
Fluorescent powders: Powder containing fluorescent chemicals that is applied to a surface to reveal latent prints; used in conjunction with an alternate light source.
Impression evidence: Objects or materials that have retained the characteristics of other objects that have been physically pressed against them.
Latent print: A fingerprint, palm print or footprint that is not readily visible under normal lighting.
Locard’s Exchange Principle: The theory that every person who enters or exits an area will deposit or remove physical material from the scene.
Other responders: Individuals who are involved in an aspect of the crime scene, such as perimeter security, traffic control, media management, scene processing, and technical support, as well as prosecutors, medical personnel, medical examiners, coroners, forensic examiners, evidence technicians, and fire and rescue officers.
Presumptive test: A test that is typically conducted at a crime scene that provides investigators with basic information regarding the compound in question.
Probative: Possessing the potential to provide details that are valuable to an investigation.
Reconstructive evidence: Allows investigators to gain an understanding of the actions that took place at the scene; a broken window, a blood spatter pattern, bullet paths and shoe prints.
Reference sample: Material from a verifiable/documented source which, when compared with evidence of an unknown source, shows an association or linkage between an offender, crime scene, and/or victim.
Trace evidence: Physical evidence that results from the transfer of small quantities of materials (e.g., hair, textile fibers, paint chips, glass fragments, gunshot residue particles).
Transient evidence: Evidence which by its very nature or the conditions at the scene will lose its evidentiary value if not preserved and protected (e.g., blood in the rain).
Unknown/questioned sample: Evidence of unknown origin; these samples could be found at a crime scene, transferred to an offender during commission of a crime, or recovered from more than one crime scene.
Forensic Evidence Admissibility and Expert Witnesses
Frye Standard:
Established in Frye v. United States (1923).
Rejected the lie detector (polygraph) due to lack of general acceptance.
The court had to decide if the procedure, technique or principles in question were generally accepted by a meaningful proportion of the relevant scientific community.
Federal Rules of Evidence, Rule 702:
Adopted in 1975.
More flexible than the Frye Standard, did not strictly require general acceptance.
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:
the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
the testimony is based on sufficient facts or data;
the testimony is the product of reliable principles and methods; and
the expert has reliably applied the principles and methods to the facts of the case.
Daubert Standard:
Established in Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993).
Federal Rules of Evidence superseded Frye’s "general acceptance" test.
Trial judges are the final arbiter or gatekeeper on admissibility.
Judges should consider:
What is the basic theory and has it been tested?
Are there standards controlling the technique?
Has the theory or technique been subjected to peer review and publication?
What is the known or potential error rate?
Is there general acceptance of the theory?
Has the expert adequately accounted for alternative explanations?
Has the expert unjustifiably extrapolated from an accepted premise to an unfounded conclusion?
Who Can Serve as an Expert Forensic Science Witness at Court?
Federal Rules of Evidence 702: Allows experts “qualified … by knowledge, skill, experience, training, or education."
Requires a significant level of training and experience.
Training plans include in-house training, assessments, practical exams, and continuing education.
Oral presentation practice is very helpful in preparing examiners for questioning in a trial.