9.2 Crime Scene Investigation & Evidence Collection
Key Takeaways
- POST Unit VII.B, Crime Scene Investigation and Evidence Collection, is a 20-hour required block, the second largest single sub-topic in the entire academic program.
- Locard's exchange principle — every contact leaves a trace — is the operating theory behind scene protection, and it applies to the officers walking through the scene too.
- Photograph in three tiers: overall establishing views, mid-range showing the item in context, and close-ups with and without a scale.
- Biological evidence must be air-dried and packaged in breathable paper, never sealed in plastic, which promotes bacterial growth and destroys DNA.
- Under C.R.S. 24-33.5-113 a sexual assault evidence kit must be submitted to an accredited laboratory within 21 days of receipt by the law enforcement agency when the victim consents to testing.
Crime Scene Investigation & Evidence Collection
Unit VII.B — Crime Scene Investigation and Evidence Collection (20 required hours) is the second-largest sub-topic in the entire Colorado POST academic program, behind only Report Writing and tied with Crowd Control plus several others combined. Twenty hours signals what POST expects: not awareness, but competent, independent processing of an ordinary patrol scene.
1. Locard's Exchange Principle
Every contact leaves a trace. The offender brings something to the scene and takes something away — fibers, DNA, prints, soil, glass, tool marks. The corollary that recruits under-appreciate is that responders are contacts too. Every officer who walks through a scene deposits and removes material. That is why scene control precedes processing, and why the entry log exists.
2. Documentation in Order
The sequence is fixed, because each step destroys some of the information available to the next:
1. NOTES (continuous, timed, contemporaneous)
2. PHOTOGRAPHS (before anything is touched or moved)
3. SKETCH (measured, with a fixed reference and north arrow)
4. SEARCH (systematic pattern)
5. COLLECTION (most fragile / most perishable first)
Three photographic tiers.
| Tier | Purpose | Technique |
|---|---|---|
| Overall / establishing | Orient the viewer; show the scene as found | Wide angle from each corner and each approach; include a street sign or address |
| Mid-range | Fix the item's relationship to fixed features | Item and a recognizable landmark in the same frame |
| Close-up | Show detail | Two exposures: without a scale, then with a scale and an evidence marker |
Photograph the scene before placing evidence markers, then again with markers. Photograph the surrounding area, the approach routes, license plates of vehicles present, and the crowd — offenders return and watch. Never delete a photograph, including a bad one; deletions are discoverable and look like spoliation.
Sketching. A field sketch does not need to be to scale, but it must be measured. Use rectangular coordinates from two fixed reference points, or triangulation from two permanent features. Record a north arrow, the sketcher's name, date, time, case number, and the measurement method.
3. Search Patterns
| Pattern | Best use |
|---|---|
| Spiral | A single focal point, indoors or a small outdoor area |
| Strip / line | Large open areas; multiple searchers abreast |
| Grid | High-value scenes; the strip search repeated at 90 degrees |
| Zone / quadrant | Structures; a room or quadrant assigned per searcher |
| Wheel / ray | Small circular scenes with a central point |
Search high and low, not just at eye level — ceilings, light fixtures, under furniture, in trash, and in plumbing traps. Collect the most fragile and most perishable first: trace, then biological, then latent prints, then bulk items.
4. Evidence Types and Handling
| Evidence | Collection | Packaging |
|---|---|---|
| Latent prints | Powder and lift on non-porous; chemical processing at the lab on porous | Lift card, sealed; submit porous items whole to the lab |
| Blood / body fluids | Swab wet stains; cut out or collect dried stains with the substrate | Air dry completely, then paper — never plastic |
| Hair / fibers | Tweezers or tape lift | Paper bindle inside an envelope |
| Firearms | Unload and render safe, documenting position and condition first | Box; do not insert anything into the barrel |
| Cartridge cases | Individually | Separate paper envelopes; do not clink together |
| Tool marks | Photograph with scale; submit the whole item where possible | Rigid packaging; never fit the tool to the mark in the field |
| Documents | Handle by the edges | Plastic sleeve, then envelope |
| Digital devices | Photograph the screen; note power state | Faraday bag or airplane mode; preserve power |
| Arson debris | Comparison sample plus debris | Clean, unused metal cans — vapor-tight |
| Wet or bloody clothing | Air dry on clean paper | Paper bag; keep the drying paper as evidence |
The plastic bag rule. Biological evidence sealed in plastic while wet grows bacteria and mold within hours and can destroy DNA entirely. Paper breathes. This is the single most commonly tested evidence-handling fact in basic academies.
5. Biological Evidence and DNA
Prevent contamination: change gloves between items, wear a mask when speaking over evidence, never use the same swab twice, and collect a substrate control from an adjacent unstained area. Touch DNA can be recovered from steering wheels, door handles, weapon grips and clothing, but it is easily overwhelmed by the officer's own DNA.
Colorado sexual assault evidence. Under C.R.S. 24-33.5-113 and its implementing rules, when the victim consents to testing, a sexual assault evidence kit must be submitted to the Colorado Bureau of Investigation or another accredited laboratory within 21 days after receipt by the law enforcement agency. Kits collected on or after June 30, 2025 must also be entered and tracked in COSAK Track, Colorado's statewide kit tracking system, under C.R.S. 24-33.5-113.5. Anonymous kits collected without a report to law enforcement cannot be tested unless the victim converts the report.
6. Digital Evidence and the Fourth Amendment
Cell phones require a warrant. Riley v. California, 573 U.S. 373 (2014), holds that officers may not search the digital contents of a cell phone incident to arrest. Seize the device, isolate it from the network, and get a warrant.
| Device state | Action |
|---|---|
| Powered on and unlocked | Photograph the screen; place in airplane mode or a Faraday bag; keep it powered; disable auto-lock if possible |
| Powered on and locked | Faraday bag; maintain power; do not attempt passcodes |
| Powered off | Leave it off |
| Computer, running | Photograph the screen; consult a forensic examiner before shutdown; consider live capture of volatile memory |
Location data. Under Carpenter v. United States, 585 U.S. 296 (2018), acquiring historical cell-site location information is a Fourth Amendment search that ordinarily requires a warrant. Video is perishable: commercial systems commonly overwrite in 7 to 30 days, and cloud doorbell footage can be shorter. Request a preservation letter the day of the offense.
7. Packaging, Marking and Transfer
Every item is marked with the case number, item number, date and time, location of recovery, and the seizing officer's initials, then sealed in tamper-evident packaging with the seal initialed across the closure. The chain of custody records every transfer with date, time, and both parties. Officers do not reopen sealed packaging in the field — reopening is done in a controlled environment, and the reason is documented.
An officer recovers a bloodstained shirt from a homicide scene. The blood is still wet. What is the correct handling procedure?
A sexual assault victim consents to testing of her evidence kit, which the law enforcement agency receives from the hospital. Under Colorado law, what is the agency's submission obligation?