Who’s at the Scene?
Police officers are typically the first to arrive at a crime scene. They arrest the perpetrator is he’s still there and call for an ambulance if necessary. They are responsible for securing the scene so no evidence is destroyed.
The CSI unit documents the crime scene in detail and collects any physical evidence.
The district attorney is often present to help determine if the investigators require any search warrants to proceed and obtain those warrants from a judge.
The medical examiner (if a homicide) may or may not be present to determine a preliminary cause of death.
Specialists (entomologists, forensic scientists, forensic psychologists) may be called in if the evidence requires expert analysis.
Detectives interview witnesses and consult with the CSI unit. They investigate the crime by following leads provided by witnesses and physical evidence.
All the while, the physical evidence itself is only part of the equation. The ultimate goal is the conviction of the perpetrator of the crime. So while the CSI scrapes off the dried blood without smearing any prints, lifts several hairs without disturbing any trace evidence and smashes through a wall in the living room, he’s considering all of the necessary steps to preserve the evidence in its current form, what the lab can do with this evidence in order to reconstruct the crime or identify the criminal, and the legal issues involved in making sure this evidence is admissible in court.