▶What is the proper sequence for documenting a crime scene?
The standard sequence is: (1) Establish scene security and access control — only authorized personnel enter. (2) Photograph the overall scene: wide shots showing the entire room, medium shots showing evidence, and close-ups of specific items (30x magnification for fingerprints, macro for trace evidence). (3) Create a diagram or sketch with measurements: the position of evidence items, distance from walls, door/window locations, and any blood spatter patterns. (4) Collect evidence in sequence, starting with trace evidence (hair, fibers) that might be disturbed, then larger items (weapons, containers), and finally blood samples. (5) Complete chain-of-custody forms for each item: who collected it, date/time, location within the scene, description, and container seal. (6) Secure the scene: seal doors and windows to prevent contamination. The sequence ensures nothing is missed and evidence is not disturbed before photography.
▶How do you collect DNA evidence and avoid contamination?
DNA evidence comes from blood, saliva, hair follicles, skin cells, and semen. Collection: use a sterile swab (supplied in DNA collection kits) to swab the evidence (blood on a wall, saliva on a cigarette butt). Place the swab in a sterile tube, allow it to dry (DNA analysis requires dry samples; wet samples grow mold and degrade), and seal the tube with tape. Document the location and date. Avoid contamination: wear gloves, change gloves between evidence items (to avoid cross-contamination), do not touch the tip of the swab, and do not sneeze or touch your face near the evidence. Many crime scenes are contaminated because technicians fail to change gloves or touch evidence with bare hands. DNA is also vulnerable to degradation: heat, moisture, and sunlight degrade DNA over time, so evidence should be stored in a cool, dry place.
▶What is the difference between a presumptive and confirmatory blood test?
A presumptive blood test (such as luminol or phenolphthalein) reacts with hemoglobin in blood and glows or changes color, indicating the presence of blood. A presumptive test is not definitive: it can produce false positives (some plant cells and chemicals produce similar reactions). A confirmatory test (such as DNA analysis or blood type) definitively identifies the blood as human and can identify the source (a specific person, or excluded persons). A crime scene technician uses presumptive tests in the field to locate blood spatter that may not be visible to the naked eye, then collects those samples for confirmatory testing. A presumptive test alone is not sufficient for prosecution; the evidence must be confirmed.
▶What is blood spatter analysis and what can it tell an investigator?
Blood spatter is the pattern of blood drops, splashes, and flows at a crime scene. Spatter analysis can indicate: the direction of travel (the tail of a drop points in the direction of travel), the force of impact (large droplets = low velocity, fine mist = high velocity/gunshot), the point of origin (where the blood source was when blood spattered), and the activity that produced the spatter (blunt force trauma creates a different pattern than a stabbing wound). A bloodstain expert can estimate the angle of impact and use triangulation to estimate where the blood source was located (the point of origin). Bloodstain analysis is helpful for reconstructing the sequence of events at a crime scene, but expert analysis is complex and subject to challenge in court; defense lawyers often argue that blood patterns are ambiguous.
▶How do you photograph trace evidence and what is a scale card?
Trace evidence (hair, fibers, glass fragments, gunshot residue) is small and easily lost or damaged. Photograph trace evidence with a scale card (a ruler or reference card) next to the item to show size. Use macro photography (close-up, high magnification) and good lighting (directional light to show texture and detail). Take multiple photographs: one with scale, one without scale for clarity, and one showing the evidence in situ (in its original location at the crime scene) before it is collected. A scale card should not shadow the evidence or cover any part of it. After photography, collect the evidence carefully (using a small envelope for tiny items, a plastic bag for larger items, and tweezers to avoid fingerprint contamination) and log in chain of custody.
▶What is the chain of custody and why is it critical?
Chain of custody is the documentation of every person who handles evidence, from initial collection to courtroom presentation. A chain-of-custody form includes: the case number, evidence item number, description of evidence, date and time of collection, location collected, name of person who collected it, signature, seal status (how the evidence was sealed), transfer dates and names when evidence is transferred to another person (investigator, forensics lab, prosecutor, courtroom), and condition of the seal each time it is received. A broken chain of custody (a gap in documentation, a seal that was opened without explanation, or evidence that was not properly logged) allows a defense lawyer to argue that the evidence was contaminated or substituted, making it inadmissible in trial. A single breach can taint the entire case.
▶What certifications and training do crime scene technicians need?
Crime scene technicians should have a high school diploma, complete a 40 to 80-hour crime scene investigation course, and obtain a certification such as NFSTC Crime Scene Investigator or IACAI Certified Crime Scene Investigator. Many technicians are police officers (and may start in patrol before moving to crime scene) or civilians hired specifically for crime scene work. Specialized certifications include IAI (fingerprint examination), blood spatter analysis (requires 40+ hours), and digital evidence photography. Continuing education maintains certification; standards for photography and evidence handling evolve as technology and legal standards change.
