When a major crime is reported, the first step of a CSI is to seal off the area so no evidence can be disturbed. Conditions of the location are documented such as lighting and weather. Evidence such as footprints, bodily fluids and latent fingerprints are collected promptly. Photographs of the entire scene are taken before major evidence items are moved. While all of these CSI activities are essential for conducting forensic analysis in a crime lab, they themselves are not part of the forensic sciences. Rather, they are procedures for collecting evidence without the possibility of contamination or losing valuable information.
However, one forensic technology does get employed at the very beginning of the CSI. Using specialized flashlights with multiple color bands or UV light, police investigate the crime scene for fingerprints, bodily fluids, blood traces and bone fragments that are hard to find in poorly lit environments. In conjunction with forensic lighting, law enforcement often uses fluorescent powder or chemical agents to reveal fingerprints and other evidence that would otherwise go undetected.
Another forensic science that happens concurrently with the CSI is the analysis of bloodstains at a crime scene. By examining the shapes, locations and patterns of bloodstains, investigators can reconstruct the physical events that caused the stains. Common examples include "projected" bloodstains that result from an object that strikes a blood source, "passive" bloodstains that drip from a bloody object such as a knife, and "transfer" bloodstains that occur when a wet, bloody surface touches a secondary surface. During a walkthrough of the crime scene, investigators often can determine the sequence of events by careful examination of bloodstains.
Forensics plays a much greater role in a CSI after evidence is gathered from the crime scene and is brought to the crime lab. Depending on the circumstances, any number of different forensic specialists can be employed to evaluate evidence, including bomb technicians, pathologists, toxicologists, video analysts and even archaeologists (in the case of mass graves and war crimes). In particular, DNA analysis (taken from blood samples or other bodily fluids) has played a major role in cracking many cases in recent years.
As useful as forensic tools may prove to be in a CSI, solving cases also depends on the ability of investigators to use inductive and deductive reasoning to make sound judgments about criminal motivation and behavior. Simply collecting a "shopping list" of all the evidence at a crime scene and running it through a forensics lab often may not provide any useful information unless investigators can integrate that into a compelling narrative through interrogations and eyewitness accounts.