Physical Evidence in Crime Scenes

Physical Evidence

• Physical evidence must be treated on an individual basis due to the unique history, circumstances, and problems of each crime scene.
• Scientific examination of physical evidence can yield significant insights into the nature and circumstances of a crime.
• A qualified evidence collector must be able to make innovative, on-the-spot decisions, not just rely on memorized procedures.

Common Types of Physical Evidence

• Blood, semen, and saliva: Includes dried bodily fluids on objects or cigarette butts and is analyzed to determine identity and origin.
• Documents: Handwriting, typewriting, paper, ink, and alterations are examined to determine authenticity and source.
• Drugs: Substances seized in violation of drug laws.
• Explosives: Devices or objects containing explosive residue.
• Fibers: Natural or synthetic fibers that can establish a relationship between objects or people.
• Fingerprints: Latent and visible prints.
• Firearms and ammunition: Any weapon or ammunition involved in a criminal offense.
• Glass: Fragments that may have transferred during a crime; includes windowpanes with bullet holes.
• Hair: Animal or human hair that could link a person to a crime.
• Impressions: Tire markings, shoe prints, tracks, and glove/fabric impressions.
• Organs and physiological fluids: Used to detect drugs and poisons; includes blood for alcohol analysis.
• Paint: Transferred paint, liquid or dried, such as in vehicle collisions.
• Petroleum products: Gasoline residues from arson scenes or grease/oil stains.
• Plastic bags: Polyethylene bags may be evidence in homicides or drug cases.
• Plastic, rubber, and other polymers: Manufactured materials linked to a suspect perpetrator.
• Powder residues: Items suspected of containing firearm discharge residues.
• Serial numbers: Stolen property submitted for restoration of erased identification numbers.
• Soil and minerals: Items containing soil that could link a person/object to a location, such as soil in shoes.
• Tool marks: Impressions made by tools used in a crime, such as a screwdriver or crowbar.
• Vehicle lights: Headlights and taillights examined to determine if they were on/off at the time of impact.
• Wood and other vegetative matter: Fragments that could link a person/object to a crime location.

Significance of Physical Evidence

• Forensic scientists examine physical evidence for identification or comparison.

Identification

• The process of determining the physical or chemical identity of a substance with near absolute certainty.
• Requires testing procedures that give characteristic results for specific standard materials.
• Requires sufficient tests to exclude all other substances.
• Simple rules cannot define a foolproof analytical scheme, each type of evidence necessitates different tests.
• Forensic scientist must rely on knowledge and experience to determine when to conclude analysis.

Comparison

• A comparison analysis subjects a suspect specimen and a standard/reference specimen to the same tests to determine if they have a common origin.
• Forensic comparison is a two-step procedure: select properties for comparison and then draw a conclusion about the origins of the specimens.
• Probability, as the frequency of occurrence of an event, plays a role in ascertaining the origins of two or more specimens.

Individual Characteristics

• Evidence associated with a common source with an extremely high degree of certainty possesses individual characteristics.
• Examples include fingerprints, markings on bullets/tools, wear patterns on tires/footwear, handwriting, and fitted broken objects.
• Examiners use practical experience to support such conclusions of common origin.

Class Characteristics

• Evidence associated only with a group, never with a single source, possesses class characteristics.
• Probability is a determining factor of class characteristics.
• Blood type is an example of evidence that can have class characteristics.
• Product rule: Calculating the overall frequency of occurrence by multiplying together genetic markers.

Assessing Physical Evidence Significance

• A weakness of forensic science is the inability to assign exact probability values to class physical evidence.
• One should create and update statistical data to evaluate the significance of class physical evidence.
• Forensic scientists must rely on personal experience to interpret class physical evidence.
• Cases are fashioned for the courtroom around a collection of diverse elements.

Value of Physical Evidence

• Class physical evidence can corroborate events in a manner free of human error and bias.
• Forensic labs should implement procedures designed to reduce access to unnecessary information.
• The value of class physical evidence is the thread that binds together other investigative findings that are more dependent on prone to human failings.
• Investigators should pursue all investigative leads, aided by scientists who can evaluate class physical evidence successfully.

Cautions and Limitations

• The weight given to physical evidence is determined by the trier of fact (usually a jury).
• Scientifically evaluated evidence can take on an aura of special reliability, but failure to take proper safeguards may unfairly prejudice a case.
• Physical evidence may exclude or exonerate a person from suspicion.
• A thorough collection and scientific evaluation of physical evidence must be a routine part of criminal investigations.
• The point at which evidence crosses the line from class to individual is often debatable.
• Experts must find as many characteristics as possible to compare one substance with another.

Forensic Databases

• A criminalist can link a suspect to a crime through comparative analyses.

Fingerprint Databases

• The Integrated Automated Fingerprint Identification System (IAFIS) is a national fingerprint and criminal history system maintained by the FBI.
• IAFIS was effectively replaced and integrated into the Next Generation Identification (NGI) system in 2014 and first became operational in 1999.
• NGI contains fingerprints and access to corresponding criminal history information for nearly 75 million subjects.
• Each state has its own Automated Fingerprint Identification System (AFIS) linked to the FBI's NGI system.
• Latent-print examiners for NGI create a digital image of the print, mark points, electronically submit to NGI system.
• The IDENT1 system in the United Kingdom links police and justice agencies and includes finger/palm print analysis, print search capabilities, verification of identities, and information sharing.

DNA Databases

• The FBI's Combined DNA Index System (CODIS) became fully operational in 1998.
• CODIS enables electronic exchange and comparison of DNA profiles between federal, state, and local crime laboratories, thereby linking crimes to each other and to convicted offenders.
• All 50 states have enacted legislation to establish a data bank containing DNA profiles of individuals convicted of felony sexual offenses (and other crimes).
• The forensic index contains about 915,000 DNA profiles from unsolved crime-scene evidence.
• CODIS identifies serial crimes, allowing coordination of investigations.
• Rapid DNA: Rapidly obtaining (90 minutes or less) a DNA profile from a buccal swab.
• The FBI permitted profiles collected by Rapid DNA into the CODIS database.
• National DNA Database in the United Kingdom has over 6 million profiles.
• National DNA Data Bank in Canada contains over 379,000 DNA profiles.

Genealogy Databases

• Forensic DNA profiles are used from crime scenes by searching unknown profiles through genealogy databases (like GEDmatch).
• Genealogy databases contain samples that are processed by commercial genealogy companies and uploaded by private citizens.
• These databases assist genealogists and have proved useful in cases where investigators have DNA profiles that are not matched to a source.

Other Forensic Databases

• The National Integrated Ballistics Information Network (NIBIN), maintained by the Bureau of Alcohol, Tobacco, Firearms, and Explosives, allows firearms analysts to acquire, digitize, and compare markings made by a firearm on bullets and cartridge casings recovered from crime scenes.
• The heart of NIBIN is the Integrated Ballistic Identification System (IBIS), comprising a microscope and a computer unit that can capture an image of a bullet or cartridge casing.
• The International Forensic Automotive Paint Data Query (PDQ) database contains chemical and color information pertaining to original automotive paints.
• SICAR (shoeprint image capture and retrieval) is a commercially available computer retrieval system for comparing and identifying crime-scene shoe prints.

NIBIN Links Handgun to Suspects

• A series of armed robberies were linked to a .40-caliber handgun through NIBIN.
• Cartridge casings from the crime scenes were entered into NIBIN.

NamUs for Missing and Unidentified Persons

• The National Missing and Unidentified Persons System (NamUs) was created in 2007 as a national centralized repository and resource center for missing persons and unidentified decedent records.
• Contains the Missing Persons Database, the Unidentified Persons Database, and the Unclaimed Persons Database.