Found on Duke University Law's online glossary, Chain of Evidence Sequencing follows this order: identification and collection; analysis; storage; preservation; transportation; presentation in court; return to owner. The chain of evidence shows: who obtained the evidence; where and when the evidence was obtained; who secured the evidence; who had control or possession of the evidence.
So why do we see so many evidence bags, both plastic and kraft (grocery like) used at crime scenes and hear of a Detective saying, the evidence has their mark? Why are they important?
A strong criminal prosecution and defense will rely heavily on the CoC of evidence. For example, police respond to a 911 call for a domestic disturbance. Upon arrival they find one victim DOA (dead on arrival) a possible murder weapon, in this case a knife and blood pooling around the body. Depending on the size of the department, Detectives and CSI (Crime Scene Investigators) are dispatched. The Detective is going to attempt to piece together the actions that took place leading up to and immediately following the death of the victim to include other persons in the home, weapons, history of the family, prior police calls, etc. With so much information to pour through and the possibility of so many "hands" on deck it would be very easy to smear or rub off fingerprints, disturb the victim's body, step on and smear the bloody footprint of the probable assailant among others.
There must be a CoC on all evidence to allow proper receipt (tagging) to include date, time, charges, and names before the evidence is sent to be studied. The evidence should have a CoC for every person, date and hour of the day from the time the evidence is collected to closure of criminal case and return of property. If there is not an "unbroken" chain of evidence, it can be suppressed and not admitted.The Detective will confer with the ME (Medical Examiner) to determine exact cause of death. If the ruling is a stabbing by a 6" serrated blade then the knife that was found at the scene will be compared to the findings. If there are smeared prints on the knife because it was mishandled or "cleaned" then that eliminates one certainty a.k.a. puzzle piece. A clean CoC will show who handled and possibly smeared the evidence or when the knife in custody was misplaced. Who misplaced it? Who had it last? What were they doing with it? This error has provided the Defense to call for Evidence Suppression or Exclusion which means no knife can be admitted in the case.
Forensic Science Lecture, Keith Murray, March 2011
If there are no discernable clear prints on the knife which caused the death, then the Detective can focus on the perpetrator having worn gloves which might indicate aforethought - pre-meditation versus a fit of anger and subsequent level of charge(s).
The Detective takes all of the puzzle pieces and fits them together to provide the prosecution (District Attorney) the tools to pursue justice for the victim and the community, an accurate CoC is an essential part of the puzzle.
A great source of information for evidence collection, processing and return is provided at http://dofs-gbi.georgia.gov/evidence-submissions Georgia Bureau of Investigations, Departments of Forensic Sciences. Also for reference, click to download the old evidence submission form to the GBI.